So let's switch your perspective a little bit. The "hero" of most blockbuster movies follow the basic path of early success, setback, then overcoming their obstacles and "winning." One could make the argument that of course they were going to ultimately succeed, they are the protagonist, the hero - there's an inherent dissatisfaction if the main character of a movie fails in the end. So if we view Thanos from the perspective of a villain or antagonist, him succeeding is surprising; it's when we view him from the perspective of a protagonist that him achieving his goal seems a given. I just booted up Infinity War, and the first line Thanos says is "I know what it's like to lose."
Thanos' belief system is changed in the movie, although it's challenged multiple times and integral to who he is comes from he beliefs being opposed. His driving motivation is having the will to carry out his belief system because at one point he didn't and that lead to his home one destroying itself. Infinity War is about him finding the will to do what he thinks is right, and his sacrifice of seemingly the only thing in the universe is not just a plot twist, but a defining moment in his character arc. It's why the final shot with him chilling on his farm with his armor put up in the form of a scarecrow, because his motivations aren't to kill people, but to balance out the universe so it can be prosperous. His character growth isn't just who he is from the first minute to the last second, it also takes into account who he was and where he came from in the exposition and subtext of the movie. And, continuing with the idea of subtext, for a while it seems as if power is a driving motivation for Thanos and his ideology is used as a justification, but by the end of the movie we see that isn't true anymore.
And again, to harp on the Thanos doesn't have obstacles part, that's just not true. Thanos has a moment of self doubt before sacrificing Gamora. Thanos almost loses to Tony and the crew, and might have lost if it wasn't for Quill, and he only barely succeeds there. Even with Thor we see him almost get killed. I get that it isn't like Bruce Willis in Die Hard where it's remarkable he doesn't die and barely is able to overcome the obstacles, or Batman in The Dark Knight Rises where Bruce is literally broken and beaten and has to claw his way to victory, but if the whole idea is Thanos is physically challenged enough or doesn't have a single decisive defeat for the climax of the movie that's not something that undermines the idea that Thanos is or is not the protagonist that just goes to show the mechanics of writing a character that is meant to be the indomitable villain that Thanos is while also writing him to be the protagonist. You can view him putting his generals in charge as a mistake or defeat if you so wish, but I think for the genre and what Thanos is meant to be for the MCU him being defeated might have hurt his image. Before getting the Stones he defeats the Hulk, and him overcoming everything else underscores not just how powerful he is, but also the will to win that he showcases. In a traditional movie, Thanos would lose in the end, but he doesn't. Just like in a traditional movie the protagonist will win, even if it costs them greatly. That's the clear parallel that you are dismissing, I think, in your attempt to categorize everything as some orthodoxy of writing when this is more akin to the debate on whether a hot dog is a sandwich. It's in the margins.
First and foremost. It's not inherent to the protagonist to 'win' at all. There are many stories where the protagonist loses or doesn't achieve his goal, or where the ending is both negative and positive at the same time. As I stressed before, usually the Protagonist is the one who is the most affected by the events and changes trough them. Who discovers a new part of himself. Thanos winning doesn't suddenly make him the protagonist. It just makes him the one who wins. Granted, the villain winning is something that doesn't happen a lot in a blockbuster movie, but that's another area of discussion and analysis.
Again. Thanos does not change, nor are his believes actually challenged nor is he forced to change them. He starts with a goal, goes on to chase that goal and achieves it without his believes or character truly being tested. It's a McGuffin chase, where the suposed protagonist gets all his McGuffins fairly easily along the way. It's as straightforward as it gets, and straightforward is what you should avoid in your protagonists (or any) story. A protagonist getting all he wants, without it changing him forever. Why would you want to see that (or tell that story)?
You say his arc doesn't start at minute one. And that's ehm... wrong. As a viewer you get presented a story with a beginning, middle and end, and you expect those to be te rightfully chosen moments to show us the arc of the protagonist. If the change has happened in the backstory, than simply said you have started your story too late. Than you have skipped the important beats of his story. For us, the story starts when the movie starts. Backstory is necessairy for understanding motivations and to set-up a character, but it is not to retroactively claim the protagonist has changed when it's not in the text of the film. We need to see the change IN the film. We need to be there when it happens. And we need to understand that it happens. IW fails on this with Thanos. Apparently his change lies before the movie begins, and his one true character moment, which suposedly is a change, is told in a way we do not perceive it as a change of character, but as a change of how we VIEW the character. (I'll go in on that again later). You know, this is my biggest issue with IW actually. It doesn't tell a story. It barely has any arcs or change in it's characters. It's allmost all plot and set-up. Your argument is a good example of this, because to claim change you have to look outside the text and into backstory. My dramaturgical question is then: why tell these events? And not those from the backstory?
You also say this: "for a while
it seems as if power is a driving motivation for Thanos and his ideology is used as a justification, but by the end of the movie we see that isn't true anymore." I highlighted the key words, because you enforce my point. The only change surrounding Thanos' character is how we
perceive him. He doesn't change, we discover new things that change our opinion on him. That's not an arc or character growth/change, those are reveals and twists. There is no text nor subtext that implies Thanos has changed his goals or intention. He always acted from a misguided 'saving the universe'-idea, not from powerhunger.
And this brings us to his challenges. His biggest one is suposedly Gamora. But like I said before, in the way it is told it is not presented to us as a big change in character or a huge challenge to overcome. It's presented to us as a reveal and a twist. I've explained it before enough, so I won't go to deep in it. But I can point you to the mirror of this scene. When Gamora asks Star Lord to sacrifice her, a similar set-up DOES illicit character change. This scene is told from Star Lords perspective, and we know he is not someone willing to sacrifice. Him pulling the trigger (shame of the cop-out) is a huge change for him. It's not for Thanos. We have seen Thanos stopping at nothing to achieve his goal, and in his sacrifice-Gamora-scene he does just that: stopping at nothing. It's not change. Him shedding a tear or having a reflective moment after it, doesn't make it so neither. It just shows is this big dude might have had a heart after all. (Changing our perception of him, not who he is)
His other challenges are physical. Physical challenges do not force a character to change his character. It's a test of power. You point to two instances he almost loses. Those are actually good examples of why he isn't the protagonist (or if he is supposed to be one, a dull one). In both ocassions he doesn't overcome these challenges because
he acts, he does because the Avengers make mistakes. Star-Lord flips out (very frustrating to watch, as it kind of reverts a change in his character we just witnessed, reinforcing my idea that in IW plot is more important than story and character) and Thor doesn't go for the head. It's not really compelling for a protagonist on a supposed Hero's Journey to hinge on the mistakes of others. No, the Avengers are the protagonists here. They have 'the biggest hill' to climb and are faced with the biggest challenges. IW doesn't tell a very good story, because it lacks or muddles the change of it's protagonists, and because the story only gets actually interesting when the Snap happens, but it is clear the Avengers are the protags. Thanos is at most a deutoragonist.
And I don't try to catagorize everything in an orthodoxy. I know very well there are more exceptions than rules in storytelling and screenwriting. But if you present me with an argument like 'it's a hero's journey Thanos undertakes', I'll hold that claim to the light of what a Hero's journey is, to see if Thanos fits that bill. And I do this with all these arguments. I'm not saying a protagonist needs to be the one whose point of view is used, or that a protagonist always needs change, or that he has to hit all the traditional beats. But I look at all of these to find a compelling argument why Thanos is the protagonist, and I can't find one. Because he doesn't fit any of these aspects, nor is IW the kind of story that justifies the lack of one of these aspects (a non-changing protagonist for instance usually features in a specific type of story, where not he changes, but his environment does trough his actions. Added to that, the believes of such a protagonists are usually challenged along the way, tempting him to change)
