You can have people lower than a 90 and they will still leave early. What you can do is have them rated between 86-89 still and just put their potential at 99. I meant to do some testing on what it takes to leave a player a long time ago for draft classes but I never got NBA 2k8. I'll do a search on here in a bit because I think I remember talking about it on here.
My reasoning behind not having a bunch of people in the 90s is that...
A) The default rosters had like 3 or 4 in the 90s if that
B) DKrause had similar in his edited rosters
C) It doesn't take a 90 overall rating for a player to be really good in the game.
D) Look at classic teams, Jordan is a 94 overall.
E) It also makes the team overalls go really, really high if there are too many highly rated players. It's easy to wind up with several teams rated higher than some of the classic teams if someone gets too carried away.