It's actually a big problem for web servers. If you consider apache for example, that has to do one thread per connection. (yes, apache still doesn't support events for websockets in 2020).

Let's say you configure it for 2000 max connections (really not much) so that's 2000 threads, so 20 GB of memory right away because the thread stack is 10 MB on Linux. It's a lot of memory and it's obliterating all caches.

You can reduce the thread stack to 1 MB (might crash if you go lower) but any caching is still trashed to death.

Next challenge. How do you think concurrency work on the OS with 2000 threads? Short answer is not great.

The software making heavy use of shared memory segments, semaphores, atomics and other synchronization features. That code is genuinely complex, worse than callbacks. Then you're having issues because these primitives are not actually efficient when contended by thousands of threads, they might even be buggy.