But, per your original post, wouldn't deplatforming then just be pushing it underground again? While letting it get to the point the ideas can spread and radicalize others?
I think once you’re to the point that something is illegal you’re fringe enough to where the threat is very small. There’s a big difference in the size of groups that say “we should do X” and people actually leaving their houses and “doing X”.
But the people saying "we should do X" help create the ones "doing X". When the crazy action you're contemplating also gets social acceptance and support from a group of like-minded people, all calling for it to be done, it stops seeming so crazy.