I do want to dispel the notion that you can't have any power going the other way. If you're looking for a pitch to drive the other way, it can be done pretty easily. Timing and the propensity for to pull the ball are the biggest reasons it isn't done more regularly. You have more power pulling inside pitches, sure. But it isn't as drastic of a difference as is being stated here. Even if it were the case that guys hit mostly singles going the other way, if you burn teams by that enough times, they are going to stop shifting, regardless of how many fewer home runs they may be giving up.
Really, I'm surprised you can shift on hitters as accomplished as Seager. As far as I can tell, they don't do it much to Cano or Cruz, with great reason. Seager though, as a guy that isn't going to hit more than 25 HR anyway, should see a shift, see an outside pitch, and freaking hammer it. He's a good enough hitter to do it.
The shift should only be effective against those big pull hitters with bad batting averages that aren't good enough hitters to do anything different. The fact that it is proving effective against the masses is a huge statement against the level of hitters coming into the league.
Hitters nowadays are akin to basketball players that are really strong going to the hole but can't shoot a lick, and are too full of themselves to think they need to learn to shoot to be effective. You pull the ball every time you come up, so the defense puts all their guys on that side of the field, and you're not going to do a thing about it? It's disappointing, to be honest.
