But it's not that large that one plus secondary pitch v an average secondary pitch wouldn't make up the gap.
Pitchers with 95+ velocity, like high draft picks, get numerous chances to make it work.
Pitchers who throw 90-91 and make it to the Show are a "self-selected" group, either they have good command and secondary pitches or they're soon gone, never to be heard from again (Tyler Cloyd).
Ten years ago, you could pump 95 MPH FBs all day and hitters would flail at them (how many starters averaged 94-95 a decade ago v now?), and RPs throwing hard just dominated. So command didn't matter as much.
The balance is shifting back toward "pitchers", sure a pitcher with 95 MPH velocity has a bigger margin for error, but how many guys can throw that hard, locate their pitches and develop a couple dependable secondary pitches (and stay off the IR - high velocity FBs put more stress on the arm). Since there are far more lower velocity pitchers, even if the same % develop command and secondary pitches, there will be more low velocity "pitchers" than high velocity "pitchers."
This creates two divergent paths, high velocity pitchers who struggle with command (because they became dependent on velocity early and aren't comfortable gearing down and using location), and lower velocity pitchers who are good "pitchers" because otherwise they never get out of AA/AAA ball.