We still have guys today that could manage that many innings without their arms falling off. Basically in the old days you HAD to be that guy to be a starter or you just didn't start, period. There are many factor that have played into starters throwing fewer innings. The advent of specialist relievers, increased reliance on the bullpen, stiffer offensive competition (more balanced and deep lineups with players who work counts and drive up pitch counts), a better understanding of the stress pitching puts on a body and a correlating desire from owners/management to protect their increasingly heavy investments. Also, there has probably been a shift away from a "gotta be a man and finish what you started" mentality where coming out of a game was seen as a sign of weakness rather than a strategic move. And let's be honest, guys just plain throw harder now. A lot harder. Walter Johnson's fastball, a pitch virtually singular in its day with regard to velocity, "hissed with danger" in the low 90's (by the best measurements of the day). The hardest throwers of the first five or six decades of Major League Baseball's history would almost certainly be considered soft tossers in today's game. As recently as 25 years ago guys who hit triple digits were relative rarities to be marveled at. Now every team has one (or two). Long story short, the game has just evolved. It's apples and oranges at this point.