I think the mitigation is "don't build a platform to attract white supremacists banned from other sites and then watch as they use it to plan a riot in the Capitol."
I would say that this hasn't been possible for at least the last 15 years.
So while your hypothetical isn't invalid in general, it's really not what was happening here with Parler.
What they said they were trying to be was not at all what they were actually doing - a common theme in extremist spaces.
But your overall concern about the concentration and centralization of our communications is legit. There's no easy answer there.