If it were like that the only things that could kill AT guns is artillery, and huge swaths of the battlefield will be complete tank dead zones. Tanks will get shot trying to push up, and infantry trying to oust the guns would be cut to bits by MG suppression or any blobs they have running around the front.
Thats quite the doom prophecy, but if man packed AT targeted the infantry you wouldn't have as big an issue. The squads are still going to kill the crew, hell manpakced AT does that already it would just take longer because of the accuracy towards the crew members. It would primarily benefit allied players as it's the axis who get heavy tanks