The problem is that for an observable such as the one above (a cold observable
aka one that contains a value, unlike a subject which doesn't (except
behaviorsubject etc but ignore those for now)) your behaviour on subscribe
differs.

If, say, you had the above observable (observable1), subscribed to it in one
place  (observer1) and mapped it to create another observable (observable2) that
doubles the value.

Then you subscribe to observable2.
You now want to trigger observable1 so it does it's "next" on the observer of
observable2 so that observable2gets the values. And then you want observable 2
to only forward the value to your one new subscriber, not any other ones.

That one is difficult since by default you're typically triggering everybody

If you now subscribe