My opinion, biggest difference between the pitchers of today compared to 30 plus years ago is a couple things.
one is that these kids are growing up throwing majority breaking balls from little league on up and it eventually catches up to them. I remember back in the days curveballs were limited in little league and in high school coaches emphasized more on fastball location and change ups, with curves being the 3rd pitch.
That why guys like Ryan, Carlton, etc had longevity.
The other reason is all the weight lifting they do now is a major cause of injuries, especially with hamstrings and abdominal strains, back then it was more about stretching and running, just keeping the arms loose and legs strong.
We can also kinda blame that these kids aren't stretched out enough early on in minors to throw 100 plus pitches a game like before, now i think they mostly max out around 60 or so in minors before being pulled and I'd bet the majority of their pitches are breaking balls for most of these starters. Why we see these kids coming up and majority struggle to go 5 innings and can pitch a full season.