300 Wins

The Baseball Primer’s Chris Dial argues that the 5-man rotation is innocent of charges of killing off the 300-game winner; in fact, he argues that the 5-man rotation may have helped guys get to 300. After all, as I’ve noted before, Clemens could still be followed by Maddux and Glavine, especially if Glavine can get out of Queens by August.
Dial starts with the obvious: only three 300-game winners started their careers between from 1920 and the mid-1960s. The 300-game winner was thus rare before the mid-60s.
There’s a bunch of factors at work here, and clearly a larger one is the fact that modern pitchers don’t go as deep into games. But Dial says in the comments:
The 300-G winners from the 60s didn’t get 40 starts per season over their careers. Okay, Niekro and his knuckleball would start more often. Seaver *never* started more than 36 games in a season. There would be 3-5 pitchers each season that got more than 36 GS, but not one per team or anything. By 1974, the 5-day rotation was in full use.
I decided to look more closely at the six guys from the 70s who made it to 300. Let’s say you capped all their seasons at 35 starts each – where does that get you? For each pitcher, I prorated Wins/Starts down to 35 starts (ignoring the fact that they sometimes made relief appearances):

1. Steve Carlton, 329 Wins
Carlton started more than 35 games in a season 9 times; prorating his starts, that got him an extra 16 wins. Net: 313 wins.
2. Nolan Ryan, 324 Wins
Ryan started more than 35 games in a season 5 times; prorating his starts, that got him an extra 10 wins. Net: 314 wins.
3. Don Sutton, 324 Wins
Sutton started more than 35 games in a season just 4 times; prorating his starts, that got him an extra 6 wins. Net: 318 wins.
4. Phil Niekro, 318 Wins
Niekro started more than 35 games in a season 9 times; prorating his starts, that got him an extra 15 wins. Net: 303 wins. But then, in the late 70s, Niekro was starting every third or fourth day in what was otherwise an irregular rotation; none of the other Braves pitchers started 40 games a year in those days. If Niekro were pitching today, he might do the same thing.
5. Gaylord Perry, 314 Wins
The one beneficiary, and the one guy in the group who’s almost as much a 60s as a 70s pitcher. Perry started more than 35 games in a season 10 times; prorating his starts, that got him an extra 18 wins. Net: 296 wins.
6. Tom Seaver, 311 Wins
Seaver started more than 35 games in a season just 4 times, and it was 36 each time; he really was in a 5-man rotation, albeit one that worked around his schedule. Prorating his starts, that got him an extra 2 wins. Net: 309 wins.
CONCLUSION:
It’s true that the 300-game winners of the 70s mostly got there without much help from the 4-man rotation, but they did each get a few extra wins they wouldn’t have otherwise had.