Sure, the extra years would help the vast majority of players (I.E. James Young), but I really don't see why you shouldn't be able to enter the draft until 21. That just throws tons of support behind the sham that is college athletics, and punishes the players who are ready before 21 unnecessarily.
ProbAbly more importantly, that kind of ruins all the investment the NBA has made in the NBDL. I agree that the current system is kind of bad for everyone, and changes should be made, but I think they need to finish developing the NBDL into a fully fledged minor league system. They should expand the draft to 3 rounds, allow players to be drafted at 18, and only allow teams maybe one or two NBA roster spots for under-21 players. Allow the under-21 draft picks to not take up roster spots unless your using your one or two designated spots.
That allows young draftees to start their professional careers, have a chance at cracking the league, gives the majority much more time to develop and doesn't endorse the god awful NCAA. I think that's the direction they're looking to go anyway, when you consider how much they've built up the D-League.