Earlier quoted context omitted.
When told that multi-threaded access is hard and messy, "X program uses threads and it's not a mess" is not an argument -- for one because nobody said X program is a mess. Threads being messy is not a transitive property. Nobody doing threads ever said that they are not messy or that multi-thread access and locking et al are easy.
I do threads and I just said that they are not messy.
Well, nobody sane then if I'm allowed the Scotsman! People also defend all kinds of dangerous constructs or practices blaming other programmers for not being good enough to use them even though statistically even the best programmers suffer from problems derived from them (e.g. buffer overflows, or in the case of threads race conditions, starvation, etc.).
And since we're particularly talking about adding threads to Javascript, here's the opinion of the creator of Javascript itself on threads:
You must be this tall to hack on threaded systems, and that
means most programmers should run away crying. But they
don’t. Instead, as with most other sharp tools, the
temptation is to show how big one is by picking up the
nearest ST code and jamming it into a MT embedding, or
tempting race-condition fate otherwise. Occasionally the
results are infamous, but too often, with only virtual
fingers and limbs lost, no one learns.
Threads violate abstractions six ways to Sunday. Mainly by
creating race conditions, deadlock hazards, and pessimistic
locking overhead. And still they don’t scale up to handle
the megacore teraflop future.
https://brendaneich.com/2007/02/threads-suck/