Except that in the modern game, if you tell a pitcher to ease up they'll get shelled and be out of the game in five innings anyway.
Welcome to the Launch Angle Era.
MLB teams aren't stupid. We're talking about the people who literally run and operate billion dollar franchises. They know what they're doing for the most part. Pitching has become this specialized for a reason: it needed to become this specialized or it wouldn't succeed any longer. Taking a pitcher out after five innings is only partially about health, most of the reason in today's game is about effectiveness. If a mediocre/decent pitcher faces a modern lineup a third (or, gasp, fourth) time, they're going to get smashed and likely exit the game anyway. Except in that smashed scenario, your team also loses 7-4.
Which is why so many of us advocate for rule changes to push baseball back toward a more exciting, action-packed direction. We can't just put the toothpaste back into the tube. Analytics exist, we have that knowledge, and things will not return to the way they were without someone actively pushing them back in that direction, probably by force of rules.