Trust is one of the most fundamental parts of any monetary system, so brute forcing hashes in this case is directly related to it.
Bitcoin can easily serve the world on 100 Mac Minis. Probably even fewer. The fact that currently people beat themselves into burning ridiculous amounts of electricity to run Bitcoin nodes is a function of the profitability of doing that. If that profitability decreases, so will the electricity burned. If I remember correctly, the protocol is designed to reduce that reward over time and unless the dollar value of Bitcoin dramatically increases, the energy waste should decrease long term.
A secondary point on energy consumption is how that of Bitcoin compares to the traditional financial transaction systems. I don't have the numbers at the moment but last time I checked it wasn't pretty for the latter.
With all that said, if PoS is proven to be as robust as PoW, it would probably be adopted by systems currently on PoW, like Bitcoin.