Some can be moved - Let's Encrypt has been a great help to this effort - but many others can't. Has there been any discussion within the Registry or Search teams about how to address these kinds of situations?
Knowing him, as long as the private keys to S3 don't leak somewhere, it's harder for me or anyone else to impersonate him and take the site down or start posting BS.
As for something that couldn't be moved, I suppose if the original source to a site was lost or corrupted and all that existed was a bunch of pages generated from a tool, it might be harder to migrate. Proxies help, but that isn't going to cover all bases.
You would have to figure out how to reverse transform that content into the original. Which might require a tool that can't run on modern hardware, which is a whole other headache to deal with.
The simpler non-technical answer is memories fade and people eventually die, so you need to plan for that too at some point and make sure somebody else can take over when that happens.