I agree on those, as these consume the device itself (unlike input_* helpers).
But those cases could just be fixed/handled by the respective integrations already. For example, when using a Switch as X, the replacement entity gets added to the device.
So from that perspective, I think “Wth can’t I add a helper an existing device” would maybe be better described as “Wth don’t Riemann sum and Utility Meters add themselves to their respective devices”.
…/Frenck