For plenty of already-in-shape people, the calories expended during the exercise are largely incidental, with the goal of exercise being to enhance or maintain some other property of their physical capacity.

I'll disagree slightly, though clearly you are correct that if your goal is to offset calories, then eating a donut negates the benefits of the exercise activity.

My disagreement is that I think exercise should not primarily be about calories - it should be about fitness. And almost all of the fitness gains from exercise persist even if you replace the calories with a donut.

Exercising for 30min and then relacing those spent calories with donuts is FAR better than not exercising and forgoing those extra calories.

that not how it works. that's not how any of this works.

the aerobics build up muscle that will always be burning calories by merely existing. a donut here and there won't make a negligible difference, as long as the weekly aerobic activity level is maintained.

You'll still have improved cardiovascular fitness even if you aren't losing weight.

Any ways, a lot of studies have shown your body has a variety of methods that attempt to counteract excess calories burned, like reduction in non-exercise activity thermogenesis.