There is also one very big difference between today and yesteryear.
Dr. Andrews' studies led him to the surprising conclusion that throwing breaking pitches had no noticeable impact on outcomes. The big issue was with the amount of use generally, and the related stress.
And that stress is much, much greater today because pretty much everyone is throwing so darned hard.
Sure, there was Walter Johnson way back when, then Feller, then Koufax and Gibson (though Koufax may be more of a cautionary tale than a counter example). As Ryan shows, every generation has its genetic freaks, guys who can just do more than the rest of us. But for generations, pitchers focused on pacing themselves to go 9, and getting maximum velocity was something that a lot of pitchers never worried much about.
But I can go around the Richmond, VA metropolitan area this spring and find you half a dozen or more kids throwing 90+ in their high school games. If you don't throw 85, you may not get on the mound much, and if you're sub-80, you either have to be a crafty lefty or maybe you will get left on JV (or cut entirely). You just didn't see this kind of stress decades ago. Sure, there were kids who threw hard -- in a relative sense. But as a guy who used to blow batters away sitting 82-ish myself, I can tell you, those guys you remember throwing smoke 30-40 years ago do not compare to what you're seeing out there today. We know much better how to tweak mechanics and condition kids to generate velocity. But there's no getting around the fact that high velocity brings with it high stress.
You can also watch old films and notice how pitchers pretty much used mixing of pitches to keep guys off balance. While everyone tried to hit spots, very few guys took the mound with the expectation of pinpoint accuracy. These days, I can watch any day's "Baseball Tonight" show and see a dozen HR's attributed to a pitcher missing his location by maybe 3-4 inches. So you're not going to see guys just loosely letting a ball fly. There is stress over each pitch, and the margin for error is non-existent. To me that's why you don't see many complete games these days. It's not because guys aren't strong; they are. They can hold their velocity through 9 innings, no doubt. But can they keep pinpoint accuracy after they hit 100 pitches? Not many, not often.
I don't think there's ever been a time when anyone expected a guy to throw 92-95 with pinpoint accuracy for 110 pitches or more. There was a time when you didn't have to, to complete a game. Now that you would have to, it's just not done much. And recognizing the evident frequency of injuries and trying to do something to stave them off is obviously judicious.