(ending spoilers)I think Falcom is sort of hitting a "soft reset" button with the character lineup, because as both CS1 and CS2 demonstrated, many characters ended up with a lot of dead emotional and plot weight, mostly just being towed around. I think a lot of players including myself would agree that characterization was one of Cold Steel's weakest points and the very large roster contributed to that.
Now that Class VII is out of the way, they can really tell and conclude Rean's story, and Class VII will be ancillary characters that will act as more of a role of "elevated side characters" that was really done well in the Sky games (Take Kevin, who does not spend the whole game with you by any margin, yet is very important, appears during important scenes, etc.). Tightening up the story and character development and improving the quality of the writing are the three things I wanted to see in CS3, and I think that making everyone leave was the best way to approach that. Trying to forcibly continue everyone's plot threads as a group would've just made the problem worse.
This is just my speculation mixed in with what I want to see of course, and is not indicative of Falcom's actual direction...
I was actually gonna say the same thing.
A lot of Class VII have no real reasons to return in a playable capacity other than simply because they were in Class VII. A few of them, sure. But not all, at least not beyond playable for a small section.