Illegal is the appropriate word when you're looking at something that's draining resources (electrical and human) in every major country and providing nothing of value in return. (I've made some nice money from crypto speculating, but that's not making the world better, that's just taking advantage of bigger fools.)

Proof of work systems, which by design compete on wasting resources, need to internationally outlawed.

If people want cryptocurrencies to keep going, they can get a move on finally fucking migrating to these alternatives systems they tell us are just around the corner. If none of the major networks are able to do so, they shouldn't exist.