Honestly, if D&D had made a few more fan service-y changes( Jon Killing the Night King, Arya Killing Cersi, etc) I think the vast majority of the viewers wouldn't have cared about how rushed the final season was.
Yes and no. I get the feeling that even "casual" fans of GOT were more invested than the average casual fan of a standard show. In my experience I haven't run into any fan that is even "good" with how the show ended. The best I've heard from these casual fans is that the show was "ok" and they just moved on without giving it much of a second thought. To me that's an incredible indictment of the show, that at best the final season isn't defended as it is just brushed aside, and has erased the good will from the previous seasons. Like Lindsey said, season 8 retroactively makes the rest of the show worse and I think even the most casual fan has noticed that.
And you're right, a few critical changes would have changed people's opinion, but not
just because they were fan service, but because they would have made more sense than what we got. Arya killing the Night King is a questionable decision by itself (not terrible, but certainly not earned either), but if Jon get's the kill it is both fan service AND consistent with what the show had been setting up. If Arya kills Cersei it, again, is the culmination of almost a decade of story telling and is far more consistent than Cersei dying in a basement while weeping. Time and again in season 8, the obvious choice was not only the fan service-y option but also the one that maintained the most internal consistency. This is why the term "subverting expectations" is used so ironically, D&D got so hung-up on subverting expectations that they hurt the show by overthinking themselves.
That's been my major point all along about season 8; the majority of the problems with the season could have been fixed/improved/avoided with simple alterations. Everything about season 8 felt like the final script was also the first draft of the script. It reminds me of what I would do as a kid when I had forgotten to do my homework until the bus ride in the morning, a "oh shit, I gotta crank this out" mess that may have hit on the points the teacher wanted but didn't do it in an effective or cohesive way. You can keep Arya getting the kill on the Night King, but did anyone stop and ask if their script for Jon made any sense? So you want to make Jamie a dedicated lover to his monster sister? Ok, but does it do his character any service by stating things like "I never cared about the innocent"? Fine, you want to keep Cersei alive until the end? Does it really do her any service to have almost no screen time aside from drinking on the balcony, and then to die in such a indirect manner? Ok so Dany is the final big bad, but have we done enough in the first 4 episodes of the season to show her slip into madness? In the end D&D badly needed a fresh set of eyes because all of these issues strike me as originating from people who were too close to the material to see its flaws. With their omniscience of the story they unintentionally glossed over everything and gave only cursory attention to plot/character development because in their heads it was enough. That throw away line that the Red Priestess give Arya in Season 2? Yes, that was enough to justify her killing the Night King because we always knew it was going to end that way. Dany slipping into madness because we showed her being upset in episode 5? Yes, that was enough because we know where the story is going.
This is the reason that I hope GOT is remembered. There are lessons to be learned from this show that go beyond the standard comments about the writing. I think there is a real conversation to be had about the appropriate role of studio involvement (or interference as some would say) with a creative team. Did HBO execs trust D&D too much and take a too hands off approach with GOT in those final years? What did HBO truly think of season 8, and when did they think it? How will HBO (and any studio) approach a situation like this in the future? How do you balance the freedom that creative types will demand, with the structured organization that can help reel them in if they get off course?
