I will agree that "Voat" is a nasty place, but I'm not comfortable with shutting them down either, or cutting them off from pay processors and certificate authorities. This kind of game tends to come back and bite one in the ass. It's the old "First they came for xxx and I said nothing".
The line is very tricky, as to what constitutes colorful metaphor, vs. explicit calls to violence. (My favorite satire on this dilemma: https://youtu.be/eg3_kUaYFJA) Regardless, the emotional energy and goals of that community on Jan 6th was extremely clear: to alter the due process of law by threat of force.
All that said, I'm quasi-absolutist on free speech issues, for the slippery-slope reasons you describe. Free speech doesn't end at 1A: it's a cultural value as well. Yet, it's also a tall order to enforce that value proactively: if a newspaper is not obligated to print every letter, should Twitter be obligated to let everyone have an account? There's a good argument for a "digital commons", and regulating tech giants as utilities; but the balance between an individual's freedom to share offensive views, and the freedom of service providers to choose to whom they offer service, is extremely murky.
I agree, that it's unfair to (a-priori) castigate free-speech-absolutist platforms; there are good-faith, civil-libertarian reasons for them to exist. Nonetheless, they inherently act as a moth-flame to specifically those people that are banned from more mainstream platforms; after all, what motivation would non-controversial communities have to migrate? I absolutely the right of such platforms to exist, but from Voat, to Gab, to now Parler, in practice they quickly skew towards conspiracies and "hate speech" and the like.
I consider SSC's heterodox "Culture War Thread" [0] to be the canonical case study in this pattern: "It's very easy to remove spam, bots, racial slurs, low-effort trolls, and abuse... But once you remove all those things, you're left with people honestly and civilly arguing for their opinions. And that's the scariest thing of all."
[0] https://slatestarcodex.com/2019/02/22/rip-culture-war-thread...