> And FWIW "opportunity cost" doesn't really show up as "a cost" in the traditional sense.

I cannot think of any forward-looking situation where opportunity cost doesn't show up. What case are you thinking of? The discussion, of course, is about a forward-looking situation.

In hindsight often one becomes more interested in seeing if the opportunity cost was paid. A common way to calculate that is to forget about cost and only look at expenses. More specifically, income minus expenses. The result of that calculation gives the opportunity realized. This may be what you are thinking of, but the opportunity cost is still there, it's just that the way of looking at it has changed.