The "magic" here should be that whichever app was last explicitly cast to an external display should be the one that keeps casting to that display until the user chooses from a different app to cast it to the external display.

I think the counter-argument you posed is the weaker of the two. You of course know you want to switch to displaying the live demo environment instead of the presentation you chose to cast to the display, but there's no way the OS can reasonably infer that, so it shouldn't.

> Here, I think this hits a fundamental weakness in Apple’s design culture: if you want to make things magical by removing options from users, occasionally you’re going to hit something that’s impossible to do without giving the user control. And if anything, Apple’s culture has been overtuned towards not providing that control, if at all possible.

Sadly true.