This attack has happened, at least twice to ETC and many others. Proof of work networks with the same hash algorithm are a threat to one another, particularly,

This attack has happened, at least twice to ETC and many others. Proof of work networks with the same hash algorithm are a threat to one another, particularly, if a network exists that is profitable enough to have exorbitant resources dedicated to mining, those resources are available to attack a much smaller network if that becomes more profitable for some period than just continuing to mine the bigger one. Proof of work then only protects the largest projects using unique hash algorithms.