Well, of course, and they WILL adjust, but it’ll take years. If you think about it, it took years for teams to start shifting, but really they should have started doing it a long, long time ago, at least against certain players. As Clawdio points out some players can hit the other way, but no one shifts against them, because they know that. You could not have shifted against Mike Schmidt or Keith Hernandez, because they hit the ball everywhere. You could absolutely have shifted Cecil Cooper, who pulled everything, but no one did.
These sorts of adjustments don’t just involved MLB players saying “I’ll do something different.” They start in Little League and high school, as players are taught differently to adjust to the strategies of the time.
Hitters have less than 1/10 of a second to determine spin, velocity and probable location of the pitch. Another 1/10 to decide to swing and when.
To some extent, hitting is reflexive and hitters spend their whole life honing those reflexes, muscle memory and timing of the swing.
Just altering the timing of a swing from pull to opposite field means delaying the swing by roughly 1/00th of a second.