Well, that's not really true, is it? Every transfer window sees players go from a lower division to a higher one, sometimes even non-league to the league (there was that guy who went from burgess hill to a premier league side recently), players who go on to excel at the new level, displacing the players already there. If every player that could cut it at a higher level was already there, this wouldn't ever happen.
There are loads of talents that could do well in the championship or above but for one reason or another find themselves plying their trade part time. Maybe they didn't have the right attitude or opportunities/relationships to get into the system as a kid. Maybe they were written off because of their size. Maybe they just took their time to develop. Maybe they were never used in the right system/position to show what they could do.
True.
But the problem is spotting them (as opposed to those who are playing at that level because that is the highest level they can ever aspire to). We did it with Zamora and Peter Ward, but it doesn't always work that way!