If you spend any time following the financial markets, you're likely familiar with the term "bubble." In not a nice way. An apparently new way to become well-to-do becomes popular. More people (who don't necessarily know the entire backstory) enter the field. They're all making money. Until, suddenly, they're no longer well off. The bubble popped.
Regardless how far you go back with baseball, bullpens were handled differently then. I claim the early-1970s as my kindergarten, with 1969 being my preschool. Starting pitchers were expected to pitch until they were pinch-hit for late In the game. Complete games were a regularity.