Your concept of caching is right. However, there are a few key elements that you missed.
The cached content does not last forever in the nodes; they might get “erased” at any time.
Dynamic content is never cached. Almost all sites have dynamic content, in fact, HTML is considered dynamic as its not cached by default. You could work around this and cache it, however, you must be careful as you could desync your site content and get odd results.
You might be interested in Cloudflare always online. To be fair, it can be helpful in some scenarios
Thanks for the details. I realized caching could only last for a finite amount of time, but I just thought surely it would give the impression the site was still up and running for more than 10 minutes.
In my case, one site is pmwiki-based and the rest are Wordpress. They were all completely dead and showing “host offline,” not just partially loading.