In our case, it would mean a 3 times greater salary than BayernligaSeriously? You want to argue the competitiveness of Bayernliga with the PL? 5 different winners in the last 7 games compared to 1. PL had a CL finalist in the last 2 years, with having every finalist of the Europa and CL this year. The quality of the two leagues in it's current state is incomparable. Your argument is essentially Sancho should stay in a comparatively weaker league, a league that has had the same winner for the past 7 years, because it's better for his development somehow, and that, despite it having the same winner for the past 7 years, it's easier to win trophies, despite Dortmund winning less trophies than United in those 7 years. Sound.
Sure, if you want to make 'desired improvement' qualifable first. You'll also have to qualify how the Italian and German leagues are 'more technical leagues to hone craft.' Pop that in a metric as well, if you please.
If ifs and buts were candies and nuts.