For example, if you have a web socket, and you get a close event, I'm proposing that we just complete it for the user. This of course means identifying different things that are like this and handling them appropriately.

const socket = new WebSocket('wss://whatever');

socket.when('close').subscribe({
  next: () => console.log('closed'),
  complete: () => console.log('done'),
});

AbortSignal's abort is another one I can think of off the top of my head.

Alternatively, users will just have to add a take(1) to them.

But this does illustrate another advantage to Observable over addEventListener. It also causes the subscription to teardown and clean up, which can cascade through flatMap and other composition.