To be fair, in recent-ish times, the only young player who didn't get enough of a chance at us but went on to prove himself at our level is lawless. Even then you're going back 10 years. Could have been others but he's the only one that comes to mind, certainly not a common occurrence.
I'd rather see us finding a way of keeping the likes of mckinstry, mcalear, etc. We know we're re never going to sign an international player, but we can develop one... but more often than not our top youth are poached before they reach the first team. Kind of makes you wonder "whats the point?" And I can see why falkirk (for example) have done away with their youth academy... however I'd never support us doing that. It is still pretty successful in spite of that.