i dont think it had a negative effect on sales, but it definitely didnt help much either. open worlds, sometimes, sell a lot of because of 1. the initial wave when a good open world is launched is huge, and massively affects wom (like botw, rdr2, elden ring); 2. a very interactive open world makes way for tons of content being made, which helps the longevity of a game
I think you may be on to something there. Think about social media engagement with things like Elden Ring, like people posting videos of them randomly finding the elevator to Siofra River or even just "wtf there's a cave here" clips.
A lot of modern games seem to pop off when there's enough dynamism to them to make a moment feel like your own. Elden Ring had it's impeccable sense of discovery, TOTK had its physics and building systems, BG3 had it's choice and consequence and reactivity. Rebirth doesn't have anything like that. Notice how so much of the social media discourse for Rebirth is around shipping. Maybe part of that is because its something you actually have more genuine control over via the affection mechanics.
The whole game, from it's open world to its combat, is very rigid. Most players are going to have the same experiences
Let's just give a hypothetical: What if, instead of marking manasprings or specific combat encounters on your map to get currency to buy materia from Chadley, Materia were more often placed in the open world, and gave a glow that could be seen from a distance. So I see a materia glowing in the distance, I need to figure out how to get there. I make my way to it and, as I approach, oh shit, three tonberries attack me out of the blue, oh shit. I defeat them and get a cool new Materia (thus horizontal progression) for doing so. So now I get a small little story of my own in which I came across a cool, unmarked secret, foudn my way to it, had a fight against an iconic enemy, and got a good reward for doing so. Sure, plenty of people will find it and have that same story, but the fact that it was self-driven gives the player some ownership over it.
I don't know if that kind of stuff would have made it pop off on social media, but I can say that I would have found it more engaging, and I would have loved to have seen and heard about the things people found while exploring. The open world discussions as it is are going to be more about if you did enough homework to get X materia.
Rebirth does fall into that big open world description but it's actually one of the best open world games (technically multiple big open regions) I've played in many years, so it should be selling great if it was "open worlds that are decently integrated sell" because Rebirths world is awesome and it doesn't even force you to do any of it, if you choose to ignore it and just play for the story, so it doesn't seem like that would be what's stopping some people buying it.
As someone who's enjoyed a few open world games over the past few years, I personally feel like Rebirth's is one of the lesser ones I've played in a while. It's beautiful, for sure, but the way it delivers content was actively detrimental to my immersion within it.