For the purposes of hashing, double SHA256 is very far from being "broken" or drastically accelerated. If there's some ultra-fast chip that increases hashing by a factor of million, there are still enough of non-zero bits to have unique block hashes. And if, by chance, two blocks have the same hash, it does not break anything at all: each block has a unique serial number ("height") anyway. Hash is only used as a proof of work for a given height.