Back in the day when there were only 16 ML teams, it was not unusual for players to spend 4 to 5 years in the minors. Also, back then, if a pitcher's ERA was over 4, he was usually sent back to the minors. I get it that every major city wants a pro baseball team, and baseball is making a lot more money now then then, but it has definitely been detrimental to the quality of the game.
Again, this situation is also true for pro football, pro basketball, and pro hockey.