That's a huge change and that would be a long discussion (I think this may have been discussed already?). Such change has some merit and some problems. Anyway, I have understood the BB's don't want to create useless players so this probably won't happen. Even now many drafted players are almost instantly fired.
BB wouldn't have to create useless players, it could be made so that the players that aren't drafted simply don't get created.
So the draft is a list of info, and then at the draft this info gets translated in to a player(not sure if this is how they do it now).
Basically what I am saying is that the players that are not drafted can simply be deleted from the system as if they never existed.
Having extra players would allow the skill level of 19 year olds to be higher, but also doesn't gaurantee that you get a great draft pick, and hence doesn't just give everyone better draftees. But it still means that if a great 19 year old is scouted then even a team with the first pick would consider choosing the player.