I thought of that Dijsktra quote too. It sheds light on what's wrong with the OP's proposal: in return for less nesting, it worsens the conceptual gap between the program (in text) and the process (in time). A poor trade.

I've found that nesting to indicate asynchronous control flow can be quite a good device for keeping the program text closely related to its dynamic execution. (It's true that such code is hard to read in JavaScript, but I don't think that's because of nesting per se.) It allows you to lay out synchronous logic along one dimension (vertically) and asynchronous along another (horizontally). I hypothesize that there's a quite good notation waiting to be brought out there; unfortunately, such experimentation tends to be done only by language designers in the domain of language design, when it really ought to be done in the context of working systems—and that's too hard to be worth the trouble unless you're programming in something like a Lisp.