1905: 338 2/3 innings. 3-0 with a 0.00 ERA.
1911: 307 innings. 1-2 with a 2.00 ERA.
1912: 310 innings. 0-2 with a 0.94 ERA.
1913: 291 innings. 1-1 with a 0.95 ERA.
These are the stats of a pitcher from the turn of the century, compiling nearly twice the innings per season than the average modern day pitchers does, and still he was able to attain some of the best postseason statistics of all time.
By the 1960s baseball had shifted to a 4-man rotation, typically with a spot starter/long relief man and, in some cases, a multi-inning closer (think Hoyt Hilhelm or John Hiller). Even with these changes, pitchers regularly threw well over 200 innings each season, many times topping just over 300 as Bob Gibson did twice.
But during this time there were no pitch count considerations. And while injuries were down from the old days (when pitchers threw excessive amounts of pitches included in those high total inning counts), the reduction in afflictions was wrongly attributed to the expansion of rotations thereby limiting a certain amount of fatigue (far less than pitch counts) therefore limiting injuries at minimal efficiency, but enough to push the logic in the wrong direction.
Bob Gibson's career also lasted 17 years for a total of 3884 1/3 innings. That's an average of just over 228 innings a season, a number lowered by his first two seasons (75 2/3 and 86 2/3 IP) and his last (109 IP). But even using the slightly misleading 228 number as an average consider that the leader in innings pitched this season was Justin Verlander at 240. One of baseball's best pitchers, 26 years old and in his prime, and yet that 240 number was about 40 more innings than he has ever thrown in his career. Baseball has developed such a phobia about injuring their bigger, faster, and stronger young pitchers that they do things like cut off Joba Chamberlain after 157 1/3 innings (and maybe rightly so as he was obviously tiring towards the end of the season). Oh, and as for Gibson, in his three World Series appearances he was 7-2 with a 1.89 ERA and 92 Ks in 81 innings.
So what is the conclusion that can be drawn? Either the way modern managers handle their pitchers is ridiculous and unnecessary or players are not, in fact, bigger, faster and stronger today. And forget the argument that because Mathewson (or Gibson) didn't throw as hard as a Verlander or a Sabathia there was less wear and tear on their arms back then. I'll give credence to the argument that Justin Verlander and CC Sabathia likely throw harder than Christy Mathewson or, say, Mordecai Brown. But a few miles per hour does not explain the logic behind decreasing the innings pitched (by 100-200 innings a season), frequency of appearances, and number of pitches thrown in those appearance of these supposedly well condition, nutritionally supreme, and evolutionarily advanced pitchers. Nor does it explain the controversy raised by the idea that a manager would want to throw only his 3 best pitchers "on short rest" in the most important series of the season thus far.