> Being underground means it has less visibility to the mainstream and is thus less likely to influence people.

But you have this backwards, the point of being pushed into the mainstream is to create crises by which to attack these platforms/sites.

They weren't made mainstream by the underground, they were made mainstream by news orgs looking to attack any venue or message their political opponents have as racist/murderous/etc.

They won't be driven underground to not influence people because the point is to make them boogie men for political points. This shooter used 8chan, therefore. He also used twitter and facebook and whatever else, but that isn't good for attacking people on their politics using the bodies of dead innocent people.

These communities were already underground and the news orgs didn't like it. Now there's a spotlight on them and you don't like it. The idea is to create an untenable position for those sites.