There is a mechanism that works very well and reliably. It involves manually copying an exported backup from the old phone to the new one, and entering a 16 digit (IIRC) passcode. Wheter you consider that easy or not depends on you. For me it was a 5 minute procedure

Right, I used that procedure once, it's completely inadequate. It relies on having access to the old phone, knowing how to get files off it (and onto a new phone; both of which probably assume you know how to navigate the filesystem), and you basically need to follow documentation to do it, it's completely undiscoverable (maybe that last part has changed).

All of which is completely unacceptable in 2021 for a product meant for a large audience. Messaging is integral to people's lives, to the point where people keep 10+ year old phones because they have messages on them from people that passed away and they can't figure out how to move the messages across or to a new system. As much as it pains me to say, there just aren't any production quality alternatives to WhatsApp that can take over. And don't even get me started on Element/Matrix...

this also highlights that somehow it's ok to not be able to easily extract files out of your phone. it's maddening.

You have a point, but one should point out here that WhatsApp makes this easy only if you stick with the same type of phone... if you switch between Android and iOS you're completely SOL with WhatsApp. With Signal on the other hand you can use the (admittedly non-trivial) procedure mentioned in sibling in either case.