What if that content isn't there tomorrow?

Like others have said, it's just an attempt to make selfhosting some things prettier and more user friendly while also giving you a system you could daily drive.

Honestly if you're even a little bit familiar with Linux, I would say just install Proxmox instead. It's such a good tool that will make experimenting and learning so much easier. If you still need docker then install portainer in an LXC or VM and off you go

What are your thoughts on Kagi? How does it compare to Google/Bing/etc?

Presumably they wouldn't necessarily know the 2nd server even exists if they're just hitting the reverse proxy. As long as both the reverse proxy and the origin server aren't under the same VPS provider I would expect that to be fine.

Reverse proxy gets targeted, shut down, you spin up a new one somewhere else and keep going.