They didn't. Which is how you could see a generational pitcher like Dwight Gooden come up and throw something like 800 innings and 30 complete games by the time he was 21 and then never be the same again. Or Mark Fidrych in the 70s, who threw over 20 complete games as a rookie and then pitched maybe 20 games in his career after that. Everybody used to just shrug their shoulders about it and move on.
Cox rode Steve Avery harder than either Smoltz or Glavine in the early 90s and you see how that turned out for him.