Is it really the case they "have already learned to scale down from threads"? I'm obviously not well-versed in browser internals, but here's the very first sentence of "Threading and Tasks in Chrome":

> Chrome has a multi-process architecture and each process is heavily multi-threaded.

https://chromium.googlesource.com/chromium/src/+/master/docs...

Either way, I think browsers have a very specific scenario, where there isn't much interaction between the tabs, and they need to be well-isolated from each other. I don't think that's a good reason for everyone else to move away from threads.