Bringing through young players is particularly difficult if you're upwardly mobile as a club, especially if you're chasing successive promotions. If we'd signed some only-ready-for-the-bench young players at the start of last season, they'd be chasing something like "excellent NL player standard" to get into our team as a minimum. And then we get promoted, and the target becomes "excellent L2 player standard". Let's say the player succeeded and reached "excellent NL player standard"... their place in the squad won't have changed, and they'll need the potential to go further still, but without the first team experience.
I guess the good thing with young players with potential on sensible wages is you can always move them on relatively easily if the club's growth outstrips theirs... there's going to be demand at the level below, for free transfers at least.
The calculation changes a bit once you reach stability and expect to be playing at the same level for more than one season.