To clarify: I don't think proof of stake is perfect.
I think the original pie in the sky dream, as hinted at in the Bitcoin white paper, was for decentralization through anyone (all the way to students in their dorms) being able to participate in consensus.
I don't think proof of stake gets things all the way there, but I think it does get closer to a linear ROI curve, compared to how proof of work is right now.
But while products are moving down those stages, more wattage-efficient ones are entering at top.
I think you're right that ROI curve will get closer to linear (utility costs aside), but I don't think it's going to be a very fast progression.