I'm not sure how much of this is true. Pitchers got hurt a lot in previous generations, we just remember the ones that didn't. Previous generations didn't have TJ or TOS, they just had bum shoulders and elbows, and either didn't pitch again or became junkballers. Another difference between today and older generations of baseball, is that pitchers are max effort today, when previously they would coast more during the game. Coasting doesn't work anymore, hitters are just so much better, at all positions. While there is nothing magical about 100 pitches, there is some pretty solid work on what happens after 115-120 and the likelihood of injury following that many pitches. There is also great work on diminishing effectiveness of a pitcher each time through the order. Not all pitchers are the same, obviously, but these are pretty accepted frameworks from which to operate.