Whether or not there's an opportunity cost does matter, but why should this mean that the mining algorithm "paying off anyway" mean it can't be used? If the mining algorithm has an independent payoff whether or not you mine on the main chain, then there's still the same opportunity cost of the mining rewards.
With a PoW that has "external rewards", the cost of attacking the chain falls in proportion to the external reward. You are correct that it does not fall to zero.
