If you changed the ability to be a passive decap that would be unbalanced as people run around the map cutting territories like no other.
I am not sure this would be the case. You would need multiple vetted tanks, and you would need to split them up to achieve any real capping maneuvers. Thus playing against Soviets would achieve nice asymmetrical gameplay : they have no stock heavy hitters like the Panther, but where the US has vehicle crews and flexible forward retreat points, and OKW has superheavies and 5 levels of vet, Soviets would then have capping powah.
Or it could indeed be OP as hell; this is one of those things that we will never know, because Relic will never implement it. Secure Mode is, after all, working as intended
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