to say that defense doesn't win in the limit is the same thing as saying there is an attack that can not be defended against.

So to re-phase the question to more clearly have an answer: does there exist an attack which no one will ever be able (for all time) to come up with a defense against? (the very existence of such an attack would end the (open) internet, wholly and completely, if the only winning move is not to play...)

There will be an exhaustion of possibilities in the end. New attacks eventually run out after each surface area is hardened against those attacks.

In the limit, defense wins.

There is only one case (that i see) where this may fail. if there is a 'predicament' with the state of security: ie, if securing against attack A requires you to be insecure against attack B and vise versa (this could be a 'whack-a-mole with many different kinds of attacks' situation). But that would be 'provable'. So if such a case exists, we will know about it. And it may be true that predicaments like this could be exercised if they even can exist, we might still be able to avoid/mitigate them.

So large bets on defense winning in the end.