It's actually the opposite. With everybody shooting 3's and playing fast as possible at all 3 levels ( high school, college, pro's) basketball players move WAY more now than they did even just 20 years ago. If you just look up stuff like distance traveled and dribbles per game now compared to say the late 90's the numbers barely reflect it's the same sport being played.
Bigs used to spend most of their time in the paint playing foul line down. Now every big has to at least show on a pick n roll no matter where it is, if not just straight up switch. As bad as the Kings are look at how many times Luke had to guard in space yesterday. Imagine somebody like Ostertag or Dudley trying to do that, they wouldn't last 20 games.
Also the fact that now we know enough about some injuries leading to other injuries, so when you get the first one you sit so you don't get the second one. I imagine plenty of players back in the 80's and 90's played with calf strains and that's why they 65 on their 4th ankle surgery and haven't been able to walk straight since they were 40.