The problem with the hypothetical scecnarios is that you are only considering the people who didnt buy Rebirth, and not considering that the crazy ending of why a lot of people did, in fact, buy Rebirth.
I think the issue with a single story being split into 3 parts becomes even bigger if you are literally just retelling the same existing story, there's even less of a hook. Maybe you keep more purists onboard for the whole trilogy, but who knows, maybe it wouldn't counterbalance the people who aren't all that interested in experiencing the same story in a fancier presentation.
Also, its worth considering that, would CBU1 have put their full weight behind the project if they were just to do a straight remake? Personally.. I dont think so. and then, maybe the quality would've suffered too. Maybe Remake wouldn't have sold as well and we would have had reduced scope for Rebirth. Who knows.
We could also talk about how this entire project was probably 10 yeasr too late given the age of people who were fans of the original, a lot of them probably aged out of gaming and maybe they only picked up Remake because it was covid and everyone was home. But then, maybe the tech wouldn't have allowed certain things and we'd have ended up with a cut down product.
So, yeah, I dunno, I think its not as simple as "if they did X, then it wouldve been a net improvement". The only thing that would be a full, clean net improvement is being present on more platforms. Maybe it'd have dropped from remake to rebirth the same, but its different when it drops from 5m to 3m or if it goes from 10m to 7m.