If I understood this correctly, the new behavior cannot be customized back to the old behavior. If that's true, then that's obviously very bad. Generally, as an Emacs user, I don't just want an opt-in or a gentle transition, I need to be able to customize everything to my needs.

This is Emacs, so one presumes that it's a SMOeL (Simple Matter Of eLisp) problem.

However, it seems tasteless to impose the burden.