> And for the places like client/server-communication: This is kind of where "call/return" usually shines.

The WWW would like a quick word with you. CORBA as well, if it could get a word in.

> we already have other tools (signals, observables, effects, runes,...)

We can build them. We can't express them. We can also build everything out of Turing Machines, or Lambda Calculus or NAND gates.