If it takes you this long why are you adding extra content? Just focus on the main game and getting it out in less than 10 years.
Because JRPGs like FFVII were largely representative rather than 1:1 as modern design tends to dictate. Fans at least seem to want a modern version, and the creators want to do that modern version also. That necessitates content creation. Back in the PS1 days a hallway or one road could represent an entire city or a massive building. The world map is kind of the ultimate example of representative design. Nowadays that's just not how things work. And you can't just shove in an entire city and call it a day. You need to keep the pacing fun, so that might necessitate extra scenarios. Or a lot of wasted design.
Then there are also narrative questions of pacing, too. You need things to end at a logical point. You need things to climax logically, and that becomes different once you start fleshing out a city. Sure maybe they could put in more story but it wouldn't feel like a good ending. Or it would mess with the pacing and feel more like a part of a whole rather than a full game.
I'm not saying SE will be doing this perfectly. God knows with their track record I'm a sceptic. Maybe the content they add won't make sense. Maybe it'll still be poorly paced. I don't know. But these things don't magically fix themselves when you try to bring a game to modern design.
Now yes they could do what's essentially a re-paint sort of remake. Something like Link's Awakening, where it's just new graphics over what's largely the same maps, designs, characters, pacing, etc. But we all knew that wasn't what they're doing. That was obvious from the beginning. A lot of people seem to want the modern AAA version of this game but with t he release schedule and pricing of a re-paint version and that doesn't really work.