Been preaching this for a while, it's the Red Queen effect, as velocity increases, hitters who can't handle velocity are weeded out. And higher velocity generally means worse command and more injuries.
We're seeing this on the Phillies, our three best starters so far, Nola, Eflin and Eickhoff.
Once velo goes above 92 for a starter, command is more important, and if a starter like Eickhoff has two plus secondary pitches (a great curve and a good cutter), he can get by with 90-91 velo. A 92 FB on the outside corner knee high is harder to hit than a 96 FB down the center of the plate waist high.
To me the key pitch has always been the changeup, Hellickson's career has been based on a good changeup and little else. Neris can throw 95, but his key has always been his splitter, which is really just a changeup with a different grip. Unlike a breaking ball, there's no "spin" to pick up, which makes it hard for hitters gearing for a 95 MPH FB to adjust.
The other key is being able to throw a 4 seamer that "rises" in the top of the K-zone, and a 2 seamer that moves and sinks (like Eflin yesterday and Nola his entire career). Again, makes it harder for the hitter to adjust.
The emphasis on strikeouts has hurt a lot of young pitchers, they run up pitch counts trying to put hitters away (and overthrow and lose command), whereas good location and changing up pitches can lead to weak contact (exit speed under 95 MPH) and lower BABIP. I think we'll see more RPs with multiple pitches and cutters/changups/splitters/bottom dropping curves. Keep an eye on Leftwich for that reason.
Problem for RPs isn't just over reliance on velocity, the traditional FB/slider combo doesn't seem to be as effective as hitters learn to pick up the spin on the slider and lay off it - Morgan took a step up when he developed a solid changeup.