MLB hitters have some of the best hand-eye coordination in existence, but at what point does 100+ mph pitch velocity force human beings to just guess? How else to explain All-Stars like JD constantly whiffing at dirtballs and missing by feet, not inches? Maybe chase rates are more a product of that, than in past decades with more contact. Nowadays, it is a skill not to swing (but a major bore with constant appeals for umps to check the "check").
A decade ago, 95 mph was fast; 30 years ago anything over 90 was heat, and when Williams and DiMaggio played, they didn't have to face fresh arms coming out of the bullpen... or Negro Leagues.
Age is certainly a factor, as well. When a batter in his 30s is said to be losing bat speed, it's probably less because of waning strength, and more due to vision issues -- like mere mortals who need reading glasses. In his mid-30s, Jim Rice couldn't turn on fastballs like he did in his 20s, but could still drive a golf ball over 300 yards (shhh... he's about to tee off... and the ball's not moving).
Schwarber at 29 may just have better eyesight than Martinez at 34, and thus be a better investment for the Red Sox rebuilders.