This demonstrably works. There are plenty of porn sites out there, plenty of pirate sites, plenty of places selling illegal stuff, even Wikileaks is still up.
To your last paragraph, I think we can draw the line at "allow any transaction that you reasonably believe is actually authorized by the people who own the accounts." I don't care about being fair to payment networks. If we're going the regulatory route then we've decided that this stuff is critical infrastructure and the needs of the many etc. If individual people at the company don't like handling transaction for porn or whatever, they're free to find a new job. Different risks do make it trickier, but I think we can add some language to allow holding on to money for a certain period before paying it out, or requiring deposits, with some limits on what criteria they can use to make those determinations and how long/large the period/deposit can be.
No, they should not have the right to choose how they want to operate. All those risks you mention only exist because they choose to keep them around as an excuse to why they are necessary. They refuse to modernize their systems so it works, and they should not be allowed to keep that option.
(Honestly, IMO they should just be terminated and replaced by some public service. But if you want to keep them, they shouldn't have that kind of freedom.)
When it's the speech of the many vs the speech of the few, I'm going to err towards the speech of the many.
Also, on the matter of the latter problem — fixing that is much easier said than done. Cryptocurrencies in a way were an attempt to fix that, but governments around the world do not want this problem to be fixed. And of course they don't. The field is highly regulated, because, as I've said, having control over the payment network is not that very much different from having a physical army to physically beat you up.
As an aside, I was especially surprised by:
> We see this at work with hosting.
Do we, though? It's another topic of course, but I actually share the sentiment that the current trend is the opposite one: good old days of the Free Web are gone, and the reality is that in the days of Gmail (and other major mail providers), Cloudflare (and other major CDNs) the Internet is inherently not a decentralized structure anymore. It takes a few powerful friends to reach an agreement with each other, and everyone else has to follow.
So, anyway, what I had in mind was exactly that:
> to regulate payment processors like common carriers and require them to serve everyone equally regardless
But, as I've said, I don't quite see how to draw the line here. After all, it would be somewhat unfair to payment networks to. Both because formally they are just a business, not a commodity (which may actually be the root of the problem, I think), and they should have some right to choose how they want to operate; and also because different customers and products objectively carry different risks. So they have to be able to produce some policies, these policies just have to… to be restricted to what's necessary somehow.