Age is everything. If you're younger then your body can the take the pounding and grinding much better. Its not like MJ and co. were just resting at home 24/7 before they entered the league, they were playing high impact and intense games in college. I'm not saying LBJ will be able to play till he's 45 (although I think he might; dude's a beast), however the wear and tear he's experienced isn't as drastic you make it out to be. Tmac's a bad example because we all know how fragile he is. What about KG? He played straight from HS as well, and only started breaking down when he's already 30+. If we apply that to LBJ, he still has 5 years of being a superstar, and probably another 6 of being a roleplayer. That's 11 years of playing time left, at minimum.
