It feels like that 'massive advantage' isn't that massive though. For someone Freddie or Belt or Matt Carpenter or whoever that the shift hurts a lot, 'altering your approach at the plate' to beat the shift means slapping singles to the left side. I'm not sure that really is a 'massive advantage' unless they learn to do it every single time you shift them, but that would take a player almost creating a new swing to combat the shift which isn't a small proposition. Even then, turning Freddie into a guy who only hits singles isn't a terrible outcome for the opponent.
There was an article on this a couple years ago where a couple of guys talked about why they don't just 'beat the shift' every time they go up like fans think they should. Its just not that easy. There are very, very few guys (basically the guys who win batting titles only) for whom that is a viable option.
https://www.espn.com/mlb/story/_/id/24049347/mlb-hitters-explain-why-just-beat-shift
IMO teams should be shifting as much as the data suggests until MLB changes the rules, and I think they probably should change the rules. But I think that is the only thing that pushes teams away from shifts- not a revolution in hitting approach.