20 years seems excessive, because I don't think the game is that much longer in content than OG FF7 - according to howlongtobeat it's shorter. Yes, there are more characters, but they don't have to flesh every single one of them out to the equal extent as they did with FF7 Remake. Ensemble casts by default get less airtime, treat it like a group of singers - a three person group compared to a nine person group, for example. You don't make the songs three times longer for the nine person group, you just give them less lines per person.

The development time for mechanics doesn't need to be stupidly excessive, in the end it's simply "pick action from list, use 'currency' to cast or not, do action." Sure, there are some RNG aspects, but we already had that with Cait Sith, it's honestly not going to trigger an extra year of coding.

The world might be larger than OG, but again, treat it like the singers above - just don't go insane and you're fine.

You won't please everyone with 2D vs 3D and I'm sure that the devs will want to do FFVI in 3D to bring it to a modern audience, and I think that's understandable if it's achievable. Octopath would be a small time remake that can be done in the short term but the clamour for a 3D remake will inevitably continue. Absolutely can be done, though, and absolutely shouldn't need to take longer than FFVII Remake takes - unless working on next gen hardware beyond FFVII is increasing the time it takes to develop or something.

In the end, it should take as long as any game would take - you define a budget and then scope the game accordingly. To me, it sounds like Kitase just wants to scope it BIG which is fine by me but perhaps isn't realistic for potential sales, as VII Remake was for a far more popular/famous game so sales for a VI Remake may not be as high.