I'm not sure I agree with this (not that I - or any of us really have any idea). My gut says that right now, facing high velocity and a shift - players know they can't just "take it the other way" with much success....but also know that if they are going to beat the shift, they have to crush it. If the shift is restricted and more balls put in play go for hits, they may not feel they have to hit a HR or have a 100 MPH+ exit velocity to get a hit. If there are more holes, then contact will be more effective than it's been the past 4-5 years.
You may be correct about pitchers wanting to limit contact - but honestly - almost everyone is already trying to limit it - not sure they can try any harder. If limiting the shift puts more emphasis to better defense, then that might help counter the pitchers fear of hitters putting the ball in play.
We don't even know what's proposed, so no need to worry about what TB will do, but I have to think fielders won't be able to cross the 'line' until the ball is released from the pitcher (or maybe even until it crosses home plate). Either way, neither give you any time at all to actually move very far....you can't have fielders running when the ball is put in play.