Gensler doesn't get to wave his hand and eliminate crypto. But he does get to bring lawsuits against crypto companies alleging they violate existing law. He will have the burden of proof in those lawsuits. If he wins them he may be able to effectively eliminate crypto through application of existing law.

Edit: I realize "you have to defend it on the merits" was poorly phrased. What I meant was that I predict judges will find crypto against existing law. So if you want it to be allowed you'll need to argue for new laws. But it was a very bad way to put it.