The content has been deplatformed. /j

Same. Also most of the comments section is dated January 2021, but the post just came up as new in my RSS feed (presumably a result of whichever edit/update also set the post date to today)

Congratulations on making your first Less Wrong post about a relatively current political topic and not being downvoted! You may be the first person in history who achieved this!

Hah, thanks. At the risk of stroking my ego one too many times - can I ask you to speculate on why that might be the case?
What I mean is - I'm sure what I wrote has some meritoric value (I would've kept it to myself otherwise), but I expected this post to do similarly to how other comparable posts do on LW (first-time post, political topic, not a lot of hard analysis and abstraction, not a lot of sources linked to). Hearing that this isn't the case is surprising.

I agree with Ericf that the Americans you meet (as a non-American) are not a representative sample. In my opinion, there is still a chance your observation might be valid, only less strongly.

Possibly yes, I agree. But as I had noted responding to Ericf, some (American) decisionmaking seems to be driven by exactly the same sampling bias error I made. Indeed, the Twitter letter to Dorsey that apparently spurred him to act on deplatforming Trump was reported as signed by 300 people - Twitter employed around 4600 as of 2019.  Cancel culture seems to follow the same pattern at least sometimes - Kevin Spacey definitely got tried in the court of public opinion (and boy did he lose) faster than any accusations made it into court of law. 
I wonder if it's the sampling bias again, though - that is, only the minority of cases when people got cancelled by a minority of politically active Americans gets reported. I guess to verify, it would be useful to have a "cancel watch" to trace a large number of shitstorms on Twitter and see which ones followed up with some real-world action. But that would be a lot of handiwork. Any idea as to how this could get verified more elegantly?

I guess my question is whether the proper adjective for the "Activistocracy" should be "American" or rather "non-post-Soviet" or... what?

Yeah that's a good one. It doesn't seem to me that Western Europe is like that, but I don't have good exposure  to that culture. Hobbes definitely had more influence on European politics I think, with European governments being a lot more socially oriented (public healthcare, education, transportation...), "big" and leviathanish, compared to the US. The leviathan-ness is a very heavy factor in post-USSR politics - I wrote a pretty long comment on my model of it here [LW(p) · GW(p)]. So that would leave a lot less space for such citizen activism in both cases.
Canada and the UK could be interesting cases to verify - Canadian and UK governments have some more public initiatives I think. Have there been any high-profile Canadian/British cancellings?

The problem is not normies coming to internet, but normies judging the entire internet.

I'm not convinced these are different things, to be honest, not in the American (non-post-Soviet?) case. When normies believe it is upon them to take a moral stand about everything they see and do, because their government won't, whatever they come to will be judged. The internet included.

Which feels uncomfortable, but it kinda makes sense: you cannot have something 100% user friendly and keep the public away.

I agree, this maps onto some of my ideas about that. From what I know, Tor has become somewhat useless for "truly" illegal activity - pretty much all drug trade in Russia happens through it, drug related imprisonment rates are insane anyway - but it might become "the next internet" by virtue of being hard enough to configure and navigate for the general public. But obviously, that frontier is ultimately gonna get colonized too, so yeah, possibly it's gonna be invite only - or we're just going to be passing emails across heavily filtered mailing lists. 

The "Bible Belt Valley" sounds interesting, but first they need their own credit card company, then their own ISPs, and only afterwards their web pages will be sustainable in long term

A kind-of-sustainable alternative to this seems to exist, and this usually involves ISPs in countries that are ideologically opposed to whatever place is cancelling you. 
Consider The Daily Stormer, it's a neo-nazi blog that got under fire after Unite the Right. After hopping registrars and hosters for a year or so, they got set with a Chinese registrar/ISP and have been alive since. 
Parler seems to be following suit in some sense, they have moved to a Russian host. Reuters reports that whatever is left of 8chan, 8kun, is also hosted there.
Obviously, this will only work if whatever free speech you disseminate on that platform doesn't affect the internal politics of the country you're hosting in, so that's a factor. Here's The Daily Stomer's Andrew Anglin acknowledging pretty much that:

After the CDN ban and the registrar ban – which are both coming soon for Parler and a bunch of other MAGA related sites – they are going to be in “CHINA PLZ HALP” territory.

That was easy enough for me, as I’ve always been relatively pro-PRC, or at least not anti-PRC. I think these sites that have been promoting all of these idiotic theories about China being behind the problems in the US are going to be met with a lot less friendliness than I was.

Which leads to the question: what are the forces that drive Visa and MasterCard on "the right side of history"

Here's what seems interesting. Both the Eranet that hosts Daily Stormer and DDOS Guard that hosts 8kun and Parler are extant "clear" companies, and while I don't know for sure, I imagine they can work with mainstream payment providers. At the very least, Visa and Mastercard work in China and Russia very well in general, so it's not like the ideological opposition would cut you off per se.
I think it could be the absence of an opposing voice. There is enough people to coordinate in threatening to not use their Visas or MasterCards anymore if [insert badwrong company] is allowed to use it, but there isn't enough people to threaten the same if [same company] is deplatformed. And yes, that potentially implies a vicious circle - to potentially estabilish a platform to coordinate, you need a platform to coordinate. I think the kinda-sustainable platforms like Tor or hosting in ideologically opposing countries I mentioned above could serve as ways to break that circle.