When "Estimating the theoretical maximum" you're going to bring your assumptions to that estimate and so you're equally likely to get stuck in some local optimal point because you didn't see what else was possible, so that doesn't persuade me much. Worse, now it's a local optimal which might be entirely in your head.

There is some space for this "Estimate the theoretical maximum" as a high level insight, if the software needs to do a thing which your estimate says is impossible that's important to address up front but I think for practical software engineering you're much more likely to need profile-fix-repeat

Actually measuring is crucial and an estimate is not a measurement. I think one of the reasons many in the Handmade Community don't like measuring is that it too often ruins their "estimate" of how the crap they've written is achieving a "theoretical maximum" in favour of a boring fact that it's much worse than alternatives.

Usually the ideas it that the theoretical maximum is not a local optima though, but some ideal global optima which you know you realistically cannot reach.

That is you know the data is some size, and memory throughput is some rate, and clock cycles is some rate etc...

You are entirely correct about the line of thinking if your assumptions are down a different line of thinking such as algorithmic estimates though, where you might miss some better way of doing things.