I have just recently developed a projection algorithm for pitchers. I'm kicking it off by taking a look at what one of the best pitchers in major league history if he had not developed arthritis and had to retire in 1966. Even without playing past age 30, Koufax had two seasons with a WAR above 10 and accumulated a total 49 WAR. Here's a look at what he might've done.
Koufax was right in the middle of a brilliant career when he had to retire, and I believe he may have had the potential to be the very best pitcher of all time. I ran the numbers and if Koufax had been able to pitch a full career and retire at 42 years old he might have accumulated about 145 wins above replacement, which would've ranked 7th of all players and 3rd among pitcher's (excluding Babe Ruth who earned nearly 90% of his WAR as a batter) only Cy Young and Walter Johnson were better. Sandy Koufax had almost the best pitching ability in baseball's history.