> ...and they could make it so it stays on only of there's an external display connected

Not always, my monitor goes to sleep and turns off. At this point, macOS should turn Bluetooth off? How would I wake the computer then? I would have to open it.

> Find My would not be able to find and more importantly report a location.

That might be another reason why Bluetooth stays on. On a laptop, I'd rather have Find My working in clamshell mode (given the theft risk).