Maybe the odd one or two per generation (age group/year level) is all you can realistically expect?
You have to kiss a lot of frogs as the saying goes............
The population of Devon and Cornwall is about 1.7 million, nearly 3% of the population of England.
Argyle are the biggest club in the two counties and should have the pick of young players, so you could argue that proportionate to that, roughly one out of every thirty or so English footballers should be coming through the ranks at Home Park (meaning say, six or so players in every age group who go on to have a good pro career).
There are reasons why that's a bit simplistic, but we should surely be averaging a couple of pro standard players per year, and at least one of Premier League/Championship level. There are well over a dozen or so players from Devon and Cornwall playing in the top two divisions, which I think demonstrates that this is totally realistic.
As it is, since Adam Randell (plus Klaidi Lolos in the same cohort) we have now had three straight year groups go through without producing a single pro player.
It's massive under performance that costs the club a lot of good players and millions of pounds in revenue but because it's not really visible it gets overlooked. I fear there is quite a lot that we are getting badly wrong.