I really don't have the technical or budgetary expertise to say if it was or was not strictly necessary on either of those grounds to split it. So, if it pleases you, strike the word necessary from my initial comment.
Regardless, the fact that the game is split into three parts, for whatever reason, has apparently led to a lot of filler that has diminished the experience for at least a substantial portion of critics. Whatever the reasoning may have been, it's a flaw that detracts from their enjoyment.
As for constructive criticism, I can't really say as I haven't played the game yet. But based on the feedback the most constructive criticism would be to actually come up with 3 games full of worthwhile content. If it was truly necessary to have the game split into three parts, it was also necessary to have three parts worth of good stuff. In the Witcher for instance (also haven't played) people praise all the side quests for being well written and engrossing. So, it's possible to have a lot of side content and not have it detract from the experience. Here, again according to some critics as I haven't played the game, they apparently missed the mark.
Well a couple of things.
If you go into FF7R with the mindset that the new content is there in place of something more worthwhile, you may be annoyed and distracted in situations where maybe you wouldn't, if you consider a few things from the development side.
But what I mean by constructive is where do you think they should have ended each game? That's what I never hear from people.
Even though it's impossible for us to make any definitive assumptions about how far they could realistically go exactly, at least thinking about is still important imo, because it leads to a simple but important realization. That not every location from the original would work as an appropriate end to the first game.
I'd say few locations, even.
So where is the next best location? Is it 10 minutes before or after the budget runs out? Or 10 hours?
Then when you've decided on where to end it, how does it look for Part 2? Can it also end in an appropriate way while being a full game experience?
That includes the introduction of new characters and party members.
If you go too far in Part 1, it may leave Part 2 in an awkward spot where the most appropriate scene to end it on would make the game significantly shorter than Part 1, and not have as many interesting new characters and concepts introduced.
After all, the original game was not designed to be multiple games, so the pace was not dictated by this.
But that's something they have to consider here.
So moving the end of Part 1 further back to Midgar could solve a lot of problems (including not having to design the open world until next-gen hardware). And that's where the additional content comes in. But not only because of padding. (And they padded the original story as well). There are also things they wanted to do in the original but couldn't, or didn't have time to do, but can do now. For example, I believe we see Palmer in Honey Bee Inn in the Remake in a trailer.
In the original game, data minders have found scenes with Palmer in the Honey bee Inn that were cut from the final version of the game.
That whole lobby was cut, in fact.
But a cynical player, or reviewer, not considering many, or any of the things I just listed, may think of it in a negative light because they have the wrong idea of why it was implemented. Or why it's needed.
There are naturally also things the developers have thought of in the 20 years since the game released, that they wish they had thought of/could have done back then.
But if the player keeps thinking about how "I could have had Sephiroth skewering a snake instead", then it's going to be distracting. And perhaps not in a reasonable way if they don't consider how it should have realistically been done instead.
Based on the many comments I've read here over the years at least, no one aside from myself seems to have ever raised any of these issues.
I don't know yet how I'll like the game. But I do at least believe making it multiple games was the correct choice to get everything important from the original in a high fidelity package. Midgar also seems like an understandable choice. Though if they did it justice remains to be seen.
But I'll keep all of these things in mind, and try to have realistic expectations when I evaluate the content.