I would have preffered to see them, but contextually it makes sense they never appear since most of the FF characters live in Radiant Garden and you never need to visit there or the colleseum. Sora never asking about them makes as much sense as no one being upset when the FF X characters who were Sora's childhood friends get swallowed up by darkness and don't reappear in the endings because they're not that important to the protagonists.
You never visit, but you face time with characters from that world throughout the entire game (And we never visit the Colosseum because the game has a clear lack of side or post game content). : /
It makes ZERO sense for Ienzo and the other two to be the people you're talking two, or at least the only people you're talking to (and is especially jarring since the other two don't even have any lines). Those characters are literally just filling in a role that was occupied by Final Fantasy characters in previous games. Have Leon and co. be doing that instead of those characters. That would have been far more logical, and maybe less jarring because maybe they'd actually have gotten those voice actors.
There's no good reason for it, and the lengths people are going through to excuse it are pretty funny. We're talking about characters that exist in universe, and then cease to exist for no reason. It's clearly not a creative decision to make the game better or the story more cohesive. It's a business decision, or a compromise made due to time or budget or some other outside factor. It's like when two characters fall in love at the end of a movie, and then are broken up in the sequel because the actors had scheduling conflicts.
Obviously we don't need every single FF character to return, and we don't need constant new cameos or things like that. But, at the very least, the characters from Radiant Garden were very important and had a vested interest in what's going on with Sora and Ansem, the Organization, Maleficent, etc.
It doesn't make sense within the fiction for them to not appear, and like many other things in the game, feels like a compromise, and hurts the experience.