MJ took 1.5 years off in the middle of his prime. Kareem had to start yoga to extend his career. Magic obviously retired early and, I think we can all agree, wasn't exactly a force on defense during his playing time.
The guys in the 80s weren't exactly in tip top shape. You should know that (and I know you do) extremely well as a Celtics fan with Bird (and I love Bird). These guys are monsters now, comparatively.
Guys now are in way better shape and have to run way more than they did in the 80s. Maybe the game was more physical back then in a grab and clutch and hard foul sort of way, but not nearly in rotations and distance traveled during a game.
To your point about regular season / playoffs - I think everyone realizes that killing all back to backs and/or reducing the number of games to somewhere between 70-75 is the right thing to do.
The issue with the complaints about sitting out a healthy player is that you can't say "well, we sat X player this game because our magic 8 ball said he would tear his MCL in 20 games if we didn't". So you can't argue all the injuries that haven't occurred. However, I think after last year, it's safe to say that you're going to take 60 regular season games from Kawhi and a title versus 80 games and no title. I also think with KD's injury, no star player is really going to chance it.
Finally, is this that big an issue outside of Kawhi? It just seems like every time Kawhi sits, it's considered a systemic issue across all teams. And Kawhi has a chronic injury. He's going to do this until he retires. If you sign him, you sign up for this, period. But you are also getting arguably the best two way player in the league when he's on.