OK ... considering how long teenage prospect take to hit the majors, go back, say, 6 years or so and look at the Brewers' top 20. By my count, 14 of the 20 played in the majors, Go back to 2016, and its 13. Go back to 2014, and its 17. Granted, not all of those players have had long MLB careers, but a good portion of them have.
To answer your question about "how many SS and CF prospects do we really need?", that answer is a lot, as the players who play those positions are typically the best athletes, which makes them more versatile and makes it substantially easier to change positions later on. That, sir, is not a new conept.