What about the guys at the turn of the century who often started 50 or more games and pitched 400 or more innings? Even if you concede they didn't throw as hard that is still a lot of strain on an arm. Obviously they were tougher and more durable. As recently as 20 or 25 years ago stud pitchers would work 300+ innings. What has happened? Why are todays pitchers so fragile? Prior and Pedro couldn't close out a tough 8th inning and because of it their teams are on the outside looking in. As far as Koufax goes, his arthtritis may have been genetic, or brought on by the strain of pitching or a little bit of both.
They didn't throw as hard, they didn't have different arm angles, and the didnt have the pitches that do the most damage to the elbow and the shoulder such as the slider, the split fingered fastball, the cutter, the screwball, etc... Pitchers also didn't throw in between starts like they do now, not to mention the fact that there are probably triple the amount of pitchers in MLB that there were at the turn of the century when there were half as many teams, and way less pitchers on a roster. Plus they didn't have the sophisticated arm surgeries that they have now, so that when a pitcher blew out his arm he was done. There was no coming back. So you didn't have injury history's the way you do now. I am sure there are pitchers like Clemens who could throw 350 innings if things were done like they were back in the day.