The problem with offloading to a thread is that you can't structurally share data with another thread. So any copying of e.g. arguments can be very expensive, and awkward/inconvenient as well.
Sometimes you want to use a thread to keep the CPU from blocking (e.g. long database operations).
Also, sometimes you want to use the CPU to its full capabilities.
On the contrary. That's the prototypical use case for non-blocking event based IO. No threads needed.
>Also, sometimes you want to use the CPU to its full capabilities.
You can use all CPUs by using multiple processes. That's not an issue.
Threads are useful when you want to run multiple algorithms in parallel on the same bigish in-memory data structure, especially one that has a lot of pointers.
Something like an in-memory graph database or a desktop application that lets you work with huge files in-memory, or even complex user interfaces. For instance, I'm not convinced that the cross process bridging that React Native has to do is a great idea.
So yes there are use cases where threads are very beneficial. But on the server side it's essentially the database/analytics systems themselves, not the code that accesses them.
> The problem with offloading to a thread is that you can't structurally share data with another thread.
That's really a JS issue, not a general async programming issue, though.