[Mitranim]

No disagreement here. Rx observables are strictly more powerful than promises/futures/streams.

But why start with the superset? I don't think it's good design. Consider the perspective of a language designer. You want to start with primitives that are as simple as possible while satisfying a vast number of real use cases. One-short async primitives are an important intermediary step towards observables, it shouldn't be skipped. Observables should be implemented in terms of these.

This layered design gets you a smaller cognitive cost of entry (promises are hard enough as it is!) and higher efficiency for the vast number of cases that don't need stream-like functionality.

On top of that, one-shot primitives are conceptually compatible with blocking expressions in coroutines such as async/await, whereas streams are not.

I don't get it when people shoot for fat primitives that do it all.