Ah, I know WhatsApp warns you about setting up backups that it's not subject to encryption, but I guess I took that to be the entirety and not just media. Which is still a big gap, since in 2018 we share lots of media in chats.

In any case you are right that if you can restore an "encrypted" backup onto a fresh phone without any info from the old one, then all the bits necessary to do are held by parties who can be legally compelled to give them up.

Last I checked it was really easy to get data out of a WhatsApp backup.

No special skills needed except running locating the file, running a command and connecting using SQLite or something.