Mix throws 103 with movement.
Alvarado throws 99 and gets hit.
The increase in velocity has self-selected for hitters who can hit high velocity fastballs without movement and/or thrown in the sweet spot - because the minors are now full of these guys (see Phillies marginal prospects, most of whom throw 95+).
I think one issue is teaching young pitches not to try to throw so hard, to focus on "easy" mechanics and spin rate and not just velocity. Unless you have a 4 seamer with good "rise," throwing 98 instead of 95 isn't going to make much of a difference. But a few more inches of "rise" at 95 can have hitters miss or hitting under the sweet spot.
The key isn't velocity but batter reaction time, anything that slows the batter by say a 1/10 of a second (deceptive delivery, extension, etc) or moves the ball a few inches from where the batter perceives it, is going to be more important. Why a hard cutter at 92 can be harder to hit than a FB at 96.
Where velocity does help is a larger margin of error, if you're Nola, you have to be perfect b/c his 4 seamer over the plate looks like a beach ball in today's game. His key was always painting corners, changing speeds, etc. A great changeup makes any FB look 2-3 MPH faster b/c you can't sit on the FB. For lower velo guys, the FB is generally a "show me" pitch that is either out of the zone or gets sneaked in after a sequence of changeup/slider/curve/splitter where it freezes a hitter.
