Sure? It was both an inefficient and wasteful mechanism to secure consensus. This is why developers have been actively researching and developing how to switch Eth to PoS for years.

An electric heater runs current through a wire .. the waste product of doing so is heat. The more you 'waste' the more heat produced.

I guess the more efficient version of the same thing would be the move to using heat pumps.

Electric heaters are the classic exception in energy efficiency calculations. The heat produced is only wasted in that it will eventually dissipate. But heat is exactly what you wanted when you turned the heater on, and so heaters are often described as 100% efficient. I guess with heat pumps this logic makes less sense. It is more efficient to move heat around than to generate it.

Not quite sure how this relates to Proof of Work. People don't generally run mining rigs because they want to generate heat. The heat is almost always waste.

I've always wondered if the economics of using CPUs in heaters to do something useful and generate heat would ever work out.