The theoretical energy expended to move an object tends toward zero, without limit, as the allowed time increases. Which is almost the same thing.
(Allowing it to take 12 billion years to move an inch is rather useless, but that's the same issue with the ephemeralization argument in general: it ignores all other measures of "cost".)
There is a hard limit. If you want to get somewhere, you have to get there before the expansion of the universe moves it away forever. Our accessible universe is constantly shrinking. It's a natural law that places firm discontinuities on optimization.
This may seem silly, but microcontroller shrinking is running into a similar problem with quantum noose, which is by far the closer boundary.
The accelerating expantion of the universe also importantly bounds availible time, which makes the specific equation governing how fast optimization can be achieved relevant. For example, if the cost of increasing intelligence is sufficiently exponential past a certain point, a true singularity may never come. And if it's more like busy beaver then exponential then it may be impossible for it to ever get very far indeed.
I really want to call this argument pedantic, but so was mine. And I can't argue against your argument's correctness or validity.
(Well, as far as we know, anyway. Expansion is still the best fitting theory, but there are observable anomalies. Your argument would still win in some other form, perhaps involving Planck Length or similar.)
(Allowing it to take 12 billion years to move an inch is rather useless, but that's the same issue with the ephemeralization argument in general: it ignores all other measures of "cost".)