I found it - see my other comment, I shoulda put it here.

You have a point that I had overlooked, yet in fact the joules needed to make a pencil or anything else do go down a bit each year. Efficiency is not the only kind of economic growth, but a large contributor.

In contrast, the joules needed to mine a Bitcoin go up quite steeply as time goes on and fewer are left to mine, by design. So I don't think this is a fit.