Interesting. Fundamentally, in order to achieve async io, a continuation is necessary. Syntax may hide this (e.g. async/await), or make it apparent as in callbacks/promises. I can't blame JavaScript for not having syntax that makes async pretty.

That being said, if we're talking not about IO, but about cpu/memory bound problems... Well I'd be lying if I said it was uncommon in my career to come across people who assumed, to the detriment of simplicity, quality, and performance, that a calculation (e.g. process a list mapping op with AsParallel/parallelStream) would be aided by parallelism. That is just ignorance by Dunning-Kreuger devs who don't apply a critical eye to their own experiences.

You're focusing on performance. Performance is not the world. Parallelism is not the only reason to use cooperating processes. There are other considerations--like task fairness, and isolation.

We desperately need more isolation in software.

Absolutely. I agree.