But a cryptocurrency-backed exchange does not provide independence of a third party; so a in the pretty implausible hypothetical that you'd ban systems so gratuitously inefficient, you could make the argument that peer-to-peer transactions are OK, but that case is harder to make for exchanges.

I mean, not that this kind of ban is likely to happen; it's probably not even a good idea (unless we can be much more specific about what counts as wasteful, and what counts as comparable existing technology, and have a way to make that simple, please no patent prior art search 2.0 nonsense).

Even if cryptocurrencies had unique features that some people demand, the fact remains that these cryptocurrencies could provide the same exact features without wasting enormous amounts of energy (e.g. by using a "proof of stake" mechanism). Therefore the wastefulness can't be justified on these grounds.