In fact, this did happen last year[1]. Moxie has infamously made the argument that decentralisation reduces reliability because if any one server goes down reliability is affected (ignoring that partial outages are better than total outages).

[1]: https://techcrunch.com/2021/09/26/signal-the-encrypted-messa...

Interesting. I think it's a semantical battle between reliability (literally "can I rely on the system to be up ?") and resiliency ("if it takes a hit can I keep using it in degraded mode")