I thought the same thing, but remembered the elevator in my grandmother's building: since there are only 6 floors without much traffic, once you are in the elev

I thought the same thing, but remembered the elevator in my grandmother's building: since there are only 6 floors without much traffic, once you are in the elevator, it's all yours for your travel to another floor. The elevator can be "busy" and you can call it to your floor only when the current travel is over. You get in, you press the floor you're going to, and only then does it take new requests. So, one button works in that particular configuration. It's not absolutely efficient, but the traffic is low enough that it's not, in practice, very inefficient.