Many years ago non-league was considered a breeding ground for hungry players wanting and waiting for the chance to break into the professional game. There was a lull when clubs decided it was cheaper to import foreign talents who were no better than some English counterparts but clubs were priced out of the market for home grown players.
You can go back to the late 1980’s when Vinnie Jones and Stuart Pearce were the most high profile players to tread the pathway. Since then many have trodden the path as well but while the well didn’t run dry the opportunity became limited.
Premier League clubs began to cherry pick the very best youngsters at an early age to join their academies, although a very small percentage actually make it and many end up dropping down to non-league to start a career.