well exactly. it takes so many moments that are out of their hands to make it through to the first team squad, let alone to any sort of consistent level in the starting eleven. there is a book called 'outliers' by malcolm gladwell that takes it back to birth dates and how being born in the 'wrong' month can affect a kids sports progression because they can be placed in a group that is not as physically developed as those born a month or so ahead of them and from that point they miss draft picks etc.
man utd will look back in ten years and remember their golden youth period, as liverpool now do, ajax have and as barcelona also will in another 10-15 because quite simply no team is able to produce a constant supply of kids decade after decade. we had ours in the late 80s/90s and with the changes in the modern game it means the pressure for success forces the hands of managers to fill their teams with 'sure fire' players that can guarantee that, rather than chancing their arm with untried youngsters.
i think, tbf, more could have been made of the kids we had under our wings in the past 5-6 years but for whatever reason that didn't take place and i don't think we are the only team culpable of doing so either.