That doesn't materially change the situation, though. It can't be cheaper to a) do the computation, b) generate a proof, and c) validate the proof many, many t

That doesn't materially change the situation, though. It can't be cheaper to a) do the computation, b) generate a proof, and c) validate the proof many, many times than to just do the computation. You can cut down the number of orders of magnitude worse it is, but it'll still never be as inexpensive (let alone cheaper, as the original comment suggested).