If you save metadata back to a DNG, doesn’t it mean that every time a change is made the whole large file needs to be backed up?

Not really. It obviously makes no sense to keep backing up big DNG files after every change, but that’s a straw man argument against the format. Instead you simply backup the DNGs upon their creation, and routinely backup your catalogue. If you were to hit a catastrophe, you can reconstruct the exact state of your work by restoring these virgin DNGs and your catalogue. Sidecars don’t contain all your Lightroom work.