Earlier quoted context omitted.
That's why I like to map one of my own subdomains to the cdn service. So everything is at cdn.example.com and if I change providers, it's just a dns record change and everything is ready, even if I want to just host my own content.
Doesn't that kill the possibility that a visitor will already have common content cached when visiting your site for the first time? I know that's not the only reason to use a CDN, but it's a pretty big one.
In theory, the maximum benefit of a CDN only comes if everyone is on 1) the same version or, similar but different, 2) an evergreen version. And the latter is a big red neon sign screaming "DANGER".