[elric]

There are obviously other CDNs (or whatever Cloudflare considers itself to be these days) and other certificate authorities. They are all interchangeable thanks to open protocols (HTTP, TLS/ACME in this case). Contrasted to Signal: there are no other implementations.

[ezst]

Re: Signal, it's even worse: they are openly opposed to federation and to letting alternative clients use their server. They demand control and obedience, which has always been suspicious-enough to defeat any goodwill effort on their side. Why would I have/want to trust them when XMPP is a viable federated alternative?

[zadikian]

Signal focuses on security and privacy above all else, which they don't think a federated model can do well. Case in point, XMPP in practice is less secure than Signal but has the advantages you mentioned.

The other common anti-federation argument is spam/reputation, which is basically the reason email is becoming more centralized unfortunately, though it still survives.