Energy used to run the mint is incidental, so lower energy usage can be an attainable target if it's prioritized. The energy wasted from Bitcoin, on the other hand, is a feature and cannot be reduced without reducing Bitcoin's core functionality.
>Energy used when mining Bitcoin could also be considered incidental, the goal is to calculate hashes.
The goal isn't to calculate hashes. That is the incidental part. The goal is to have a randomly chosen node create one block every X seconds.