I was thinking about this the other day. I think it's a fascinating discussion. There is no doubt in my mind that pitchers are being developed to throw as hard as they can - all the time. This has significantly increased the number of effective pitchers at the ML level. Bullpens are full of guys throwing legit gas/sliders these days. This is why most starting pitchers aren't facing lineups a 3rd time through and/or pitching deep into games. The options in the pen are simply 'better'. This wasn't true 15-20 years ago. Back then, outside of 1 or 2 arms in the pen, even you're #3 and #4 starter, facing a lineup a 3rd or 4th time was better than the options in the pen. Of course the downside is that to throw this hard, all the time is a harder on the body and pitchers blow up more often now. But then again, there are so many more options to choose from these days, so when they blow up, you just move onto the next arm. It 100% makes sense that teams aren't investing a ton of money on pitchers these days.
That said, how did we get to this point? Obviously, there are always a ton of pitchers who are fringe ML players. They want to have a career and make some money, so they are always pushing to find a way to get a shot in the majors. As these labs started producing all these robot relief pitchers who all throw 95+ with a slider, other pitchers had to follow suit and jump on board.
I also think that hitting just keeps getting better and better too - which is what's forcing pitchers to have to continue chasing velocity and spin to survive. I think the numbers suggest that velocity remains one of the biggest factors in results and you see that now as there's very few relief pitchers who can survive throwing 90-92 MPH anymore. I think if you saw pitchers go back to throwing at 90%, you'd see scoring skyrocket. Batters are too good now - they will tee off on that. I don't think we'll see many guys like Jamie Moyer ever pitch in the majors again.
It kind of feels like a chicken and egg thing. Did batters get so good they forced pitching to adjust to throw harder? or did pitchers realize that throwing harder was their key to pitching in the majors and making millions and the batters have simply adjusted? Either way - I don't see an avenue to having a lot of SPing throwing 7+ innings again while throwing at 90% and "pitching" more. I think you might need to increase the width of the plate or something drastic to see that be possible - unless we want high scoring games and a 'good' ERA to be in the 4's.
I'd love to hear other thoughts on this.