Destroying it is called counterplay. Letting your opponent be unmolested in his strategy is going to make you lose, regardless of what it is. It's like if your opponent builds an artillery gun. You don't sit there and get bombarded: you get off your ass and you kill it.
the nature of map doesn't allow it; that's the point (at least in 3v3+). at least for caches on normal territory points right outside the bases.
do you risk a vehicle to get to it? you'll probably get detected on the way and imo, only light vehicle that can really counter caches, aka not too expensive, mobile and kills caches fast, is 222. others take too long. you don't want to risk any sort of tanks because you have to preserve.
do you use infantry? well, what infantry though, especially for allies? guards? and risk getting wiped as you try to get them out softly so you dont retreat through enemy front line? assuming they sneaked all the way across an overcrowded map.
the matter of fact is that this allows 3v3+ games to have short early game and very small mid game where armies are made up of a lot of infantry + a few tanks combined arms, and accelerate the game into late game and super late game where only heavies and few vetted medium tanks exist sparingly. - this is kinda sad imo, because i really love transitions between early, medium and late game in 2v2s.
EDIT: so the real counterplay in 3v3+ would be pushing up the entire frontline, which is what you do anyway.