Except raising prices is the only thing that has the desired outcome. If PCs use only half the wattage that they used to consume... great now I can afford to run 2 for the same price.
I cannot see that happening on the consumer side of things because consumers have generally* not looked at power consumption as a factor for which hardware to buy nor how many to run.
* "generally" because home servers / media centres / etc are often picked for their power consumption. As are mining servers (eg crypto-currency). But these are by far the exception rather than the norm in terms of consumer devices.
It only matters if energy consumption is large compared to cost of the device or otherwise total cost of ownership. If PCs use only half the wattage that they used to consume, then it doesn't meaningfully change how many computers (or how powerful single computer) I can afford, because paying for the computers themselves dwarfs the payment for running them.
