These guys are professional athletes, and the ceiling has to be WAY higher than it would be for us. I imagine hitters reach it. But even if defense is 'teachable' (and I think it is), there just is only so much time in the day. If I'm a .260 hitter (or so), I KNOW what will keep me in the league is getting that up to .280. That's where I'm going to put my time, not in increasing my defensive abilities. I'm guessing it might be like basketball. There are great shooters who simply refuse to play defense (James Harden) or don't bother with it. If you're a good shooter and a mediocre defender, which would you try to be: A good shooter and good defender? (there are 100s of those), or a GREAT shooter and a mediocre defender? I just don't think professional athletes will take the time away from their primary skills to work on minor ones. (Some will, of course; but I suspect most will not).