Those keepers do get signed if they have a good spell in Division 2 but do eventually get found out due to the sheer amount of pressure. Foster didn't last long at Utd although he had a decent career in the Premier League,I don't think it works like that Ray.
Personally, I'd rate Ben Foster as being a better keeper than both De Gea and Ramsdale.
League position of a club doesn't necessarily make a goalkeeper better than another. There's likely to be at least one keeper in Division 2 who's as good as most in the PL, but simply hasn't been picked up.