So shouldn't the only question ever be:

* Would we rather burn computing resources (i.e. on new threads) or calendar time+human resources (i.e. on code refactoring)?

Followed closely by:

* And does it even matter for our situation? (i.e. since for many situations getting every last drop of performance out of a system isn't the real obstacle; it's making it do something useful for the end-user or business)

and let's not kid ourselves that anyone trying to get maximum performance out of anything would be running JavaScript. If you need max performance, you write C, and sometimes raw ASM.

That's like saying if you aren't driving a Veyron you obviously don't care about speed so use a moped.

Having a successful async paradime helps closer to bare metal by setting a standard for how things should work. Now you can likely find more asynchronous libraries that attempt to copy the functionality of something familiar than creating something completely fresh