Look at the top ten for 1969. Juan Marichal threw 299 2/3 IP and came in tenth in IP that year.
It is odd that after the shortened 1981 season, no on ever topped 300 again. And after 1985, no one ever topped 290. A lot of that might be because were entering the Closer Era of pitcher management, and the closer often came in long before the ninth. inning. The rise of the Closer Strategy, which then lead to the specialized bullpens of today, basically killed the complete game and appears to have done a hatchet job on pitcher IP.
And now we are in full blown Death of Starting Pitching, as Tampa has shown the effectiveness of spreading out the IP with their use of openers. (Kansas City tried this a few years back, too, but was not as successful.)