You're jumping across points here, though. No one, pretty much ever, is saying velocity doesn't matter but in a hyper-competitive sport that earns players hundreds of millions for rising to the cream of the crop, any advantage will be taken, "legal" or not.
And what would you rather have, a 95mph fastball with a 2400 rpm spin rate or a 95mph fastball with a 2100 rpm spin rate?
It's pretty obvious which of those literally every MLB pitcher would choose. And that's why we're in the situation we're in right now.