Let's be real here, its not just the memory requirements, because context switching and the associated nuking of cpu caches are not free. You can go very far with it nowadays, but you can go much farther with async code, if you really need to.
And IMHO the added code complexity is not worth the trouble.
The thing is, this is just that - your opinion, generalized as The Truth. But engineering is about making the right trade-offs. Often threading will be fine, you'll win simplicity, and all is good. But sometimes you really need the performance, or your field is crowded and its a competitive advantage. Think large-scale infrastructure at AWS, central load-balancers, or high-freq-trading.
Heh? Where?
With a simulated load of ~20 users we were running over 1000 threads.
Several posts in the chain say that 20k+ threads is "fine". Not unless you have a ton of cores. The memory and context switching overhead is gigantic. Eventually your server is doing little besides switching between threads.
We had to rewrite our s3 code to use async, now we can do many thousands of concurrent uploads no problem.
Other places we've had to use async is a proxy that intercepts certain HTTP calls and user stats uploader that calls third party analytics service.
Just sayin it's not that unusual to need async code because threading overhead is too high