The genie is out of the bottle. Crypto-only underground prediction markets will always exist. I think it is better to heavily regulate legal options instead of pushing them underground. That didnt work for drugs or prostitution, and it wont work for gambling.
Except, gambling isn't illegal here - in fact, it's very common. There are lots of casinos within a few mins walk in any city in Spain. All the prediction markets need to do is comply with existing laws.
Except it generally worked for gambling for a very very long time. The existence of a black market does not mean something should be legal. Human trafficking happens, but that doesn't mean we should legalize and tax it. (extreme example I realize, but I use it to illustrate a point)