Exactly. I think the proper analyses is to look at hashing vs cost. In the beginning, it was essentially linear. For the last couple and probably the next 10 or so years at least, it will be exponential as better ASICs offer exponential speedups over CPUs. Eventually, as their capacity saturates, the curve will be linear again, based on number of devices (rather than capacity per device) and as you say on electricity.

A linear situation is much more amenable to the volunteer. If bitcoin becomes important enough, for example, Apple and the rest will throw ASIC miners and EC verifiers into their products, and the network security will be funded by its usefulness in maintaining society all together.

A bit too romantic, maybe, but I'm an optimist