Persistent undo exists to recover from an accidental write-and-quit mid-session nuking some stuff you really didn't intend to delete. If you care about its contents beyond a handful of hours, you either need to adopt proper version management, or start making backups.
Reading the PR the change was needed because the old undofile format was fundamentally broken. They considered making an undofile-upgrade mechanism, but it would've caused more issues that it would've solved. In other words: stuck between a rock and a hard place.
A duty of care also means occasionally having to break things to make it better, or else you end up being stuck with spacebar heating[0] forever. As a user it does suck, but that's the price you have to pay for using actively-developed software.
That's one use case, sure, but not the only possible one.
> Reading the PR the change was needed because the old undofile format was fundamentally broken.
That's a good reason to have a new undo file that's completely separate from the old one, and use the new one instead, and tell users "Hey, whatever undo information you had in your old undo file isn't accessible any more through neovim, you'll have to use vim if you need to get to it."
It's not a good reason for just deleting the old undo file with no warning. All the new version needs to do is ignore it, not nuke it.
"All software should be shit because most software is shit"