Well, the point is that --across-devices won't be a flag you use as a matter of habit, it's something you'll only use when rm tells you it won't cross the devic

Well, the point is that --across-devices won't be a flag you use as a matter of habit, it's something you'll only use when rm tells you it won't cross the device boundary and you realize that, yes, you really do want to delete across a device boundary. So you'll only add it in the specific cases where it's warranted, and you'll have already tried rm without it (to verify that you're deleting the right thing). Come to think of it, I don't think I've ever used rm to delete across device boundaries. It just doesn't seem like an action you usually want to take.