Indeed, although if you build from source you might very well have stuff there and upgrading won’t cause issues for you (at least, not with the upgrade itself). I don’t want to halt an upgrade that won’t be problematic. The problems come from the debs, so the test needs to be a little more fine-tuned than “is there stuff in /opt/ros/?”
Indeed, folks using snaps or docker needn’t worry, that will continue to work fine.
That will probably be true of this new feature in general, honestly. That said, I think we can improve the current situation dramatically, despite it not being a silver bullet.