In this day and age, with five-man rotations and specialized bullpens, most major league starters make 32-33 starts a season;
Hey Tony! Glavine pitched his entire career in a five man rotation. Not counting this season, he has averaged 31.75 starts a year. How are today's usage patterns any different than that Glavine was subjected to?
The best pitchers win roughly 45 percent of their starts and an inordinate number of games are decided by the bullpens in the late innings;
Starters get more money to do less, and the sizable salaries inspire shorter careers;
I'll admit that I don't see any shoe-ins to win 300 game on any major league rosters right now, but that's just because the game's strongest young pitchers -- Santana, Zambrano, Sabathia, etc. -- are so far away. All it will take for any of them or any other pitcher who has yet make the big leagues to win 300 will be to (a) pitch well; (b) pitch for strong teams; and (c) remain healthy.
These three things will coincide again, even if it takes a decade or two for it to come to pass.