It's not easy to know at 17/18 what someone's best position will be though. You might think someone's too small to be a centre half, then they have a growth spurt and suddenly they're made for it, and you've wasted 2 years playing them at full back. Or the exact opposite, a big lad looks set to be a centre half and then gets stuck at 5 10 and is deemed too small. It's not as easy as you're suggesting.
The other thing with youth football is we need to stop thinking that every under 23 is a potential first teamer. They're not, and never will be. There will always be fillers, always be lads who the coaches doubt will make a PL player even from 15 or so, but think they might make a good career out of the game. We have a duty to help those players try to make a living, and things that like have an effect on your ability to attract other promising young players, as they see they'll be looked after. As with most things, there are nuances and considerations, we can't just say 'they should do this' because that would make the current team better, without thinking about the knock on effects that might have.