I'd read it primarily as censorship risk. If a CA is under a government that wants your website taken down, they can just lean on the CA to stop renewing any certificates for it, as opposed to futzing around with ISPs or DNS providers that can only have a local impact. Or alternatively, the CAs in the future might directly use their monopoly to decide who's good and evil (especially on "integrity of the network" grounds against those judged to be spammers et al.).
At least perceived censorship risk is why the archive.is guy always uses HTTP links and not HTTPS links for his site, iirc.
I'd classify that as very low risk. If a CA's business is compromised by a government, then it's pretty easy to just switch to using a different CA, preferably in a different jurisdiction to work around the censorship.
I don't really get the argument behind using HTTP links to avoid the censorship risk with HTTPS - just provide both and get the best of both worlds. Also, using HTTP is far more prone to being interfered with in transit - I recall BT (or their ISP business department) trialling that and injecting adverts into HTTP pages. I can't recall any instance of HTTPS being censored by restricting certificates.
FWIW, a slight clarification here would be that the majority of TLS certificates are issued by CAs in the US, but the majority of CAs are not headquartered in the US.
Whilst that could be a problem, there are some CAs that aren't based in the US. I consider it a low risk issue as I haven't heard of it happening. It'd be more likely for your IP to be cut off rather than attempting to deny an SSL cert.
As you say though, the risk of revocation is low, since anyone who wants to do that can just fail to renew (renewing and revoking are different operations).
I guess it decreases the lead time of censorship. That seems pretty minor though.
???
Why?