It'd be really nice if I could find a study on this, but just because bitcoin doesn't power the majority of the transactions doesn't mean it couldn't do so without the energy needs scaling linearly with transactions. If anything I'd imagine it could become more efficient as more transactions were taking place. I see people argue this a lot and I think it's too narrow of a view on the situation.

Of course it'd be much better for us to adopt a crypto currency that uses a different algorithm that has even less energy usage. Even without that though I think it could prove to be more efficient than existing financial systems.