I think you are onto something but missing a step. Cost of hash provided is the correct measurement of how much security I add to the network. My cost of hash provided is a mix of 2 things. Up front cost for the new miner amortized over the Life of the miner and recurring for electric.
That means improved efficiency forcing miners to all upgrade is more secure than getting that hash by a magical jump in efficiency.
I think asics will hit physical limits in time. Efficiency improvements come from smaller transistors, they are already catching CPU and GPU scale which are slowing the rate that they shrink.