The key thing you're missing is recruitment. White supremacists need access to the larger platforms in order to recruit new adherents to their bigoted ideology, which is significantly harder to do if they're forced into "underground" spaces. De-platforming works -- if we make it clear that hateful ideology isn't tolerated in large platforms, then people have to actively seek out such ideologies. If such ideologies are already frowned upon in the public sphere, it makes their recruitment much more difficult.