Server applications don’t spawn threads per request, they use thread pools. The extra context switching due to threads waiting for I/O is negligible in practice for most applications. Asynchronous I/O becomes important when the number of simultaneous requests approaches the number of threads you can have on your system. Many applications don’t come close to that in practice.

There’s a benefit in being able to code the handling of a request in synchronous logic. A case has to be made for the particular application that it would cause performance or resource issues, before opting for asynchronous code that adds more complexity.

Thread pools are another variation on the theme. But if your threads block then your pool saturates and you can't process any more requests. So thread pools still need non-blocking operations to be efficient or you need more threads. If you have thread pools you also need a way of communicating with that pool. Maybe that exists in the framework and you don't worry about it as a developer. If you are managing a pool of threads then there's a fair amount of complexity to deal with.

I totally agree there are applications for which this is overkill and adds complexity. It's just a tool in the toolbox. Video games famously are just a single thread/main loop kind of application.

There’s also a really good operational benefit if you have limits like total RAM, database connections, etc. where being able to reason about resource usage is important. I’ve seen multiple async apps struggle with things like that because async makes it harder to reason about when resources are released.