Consider a door with a handicap button. The interface is: press button, door opens. Now consider a door with a motion sensor, the new interface is: walk towards door, door opens.

This is basically an example of eliminating an interface. You rarely think about these interfaces because they just do the right thing. Many very simple operations have interfaces, and you can sometimes streamline or remove those interfaces using simple mechanisms.

Adn then the sensor fails or similar, and we need some other way to open and close. Never mind some way to prop it open in case something larger needs to go through.

I actually wouldn't be so sure.

1 and 2 remind me of imperative and declarative programming languages (respectively) - telling your program how to do something versus merely telling it what to do. It's the difference between saying "x = 1" and telling your computer how to allocate sufficient memory for an integer, store that integer in said chunk of memory, and access it later.

And the first thing that comes to mind is that "you" can do declarative programming now, because someone in the past did all the imperative programming to enable it. Question is, what happens if the declarative fails? How easy is it to get at the imperative guts?