I never said more velocity doesn't matter, I said it has "marginal value", that is, the value of another 1 MPH is much smaller above 92 MPH than say from 89-92 MPH. At that point other factors become more important predictors of success.
So whether a young pitcher throws 94 or 96 on average is far less important than FB movement and command, it's easier to hit a 96 MPH straight FB down the middle than a 94 MPH four seamer with late movement on the edge of the plate. And whether that pitcher has plus secondary pitches he can command to set up that FB.
All else held equal, more is better than less, but rarely is all else held equal, and there is often some correlation, many young pitchers focus too much on velocity (radar guns are a BAD thing for young pitchers) and not enough on mechanics, how often do you see a pitcher "yank" a fastball trying to throw it harder and overthrowing it. Same with velocity and spin, my suspicion is that putting more spin on a four seamer tends to work against higher velocity.
Which is why to me, Stamina is the key to a young pitcher's future, a really strong young pitcher who can maintain velocity without laboring can refine mechanics and improve movement because he's not trying to sustain a level of velocity that requires overthrowing as his arm tires. So my ideal would be a pitcher who can throw 94 for a 100+ pitches with "minimal effort" where I can coach up command and smooth his mechanics, rather than someone pushing it up to 96-98 by using maximum effort, that's the guy I move to the bullpen.
That's why teams love big pitchers, but I think it's probably also biomechanics. It'll be interesting to see if Sixto can throw 100 pitches and maintain velocity, he may have to drop down to 96 MPH or so (boo ho ho). One reason I think Nola has been so successful is he stays within himself, he can pump it up to 95-96, but throws around 92 for most of the game and saves that extra oomph for when he needs it.