Asynchronous Everything

Agreed using procedural application environments, async is hard. But there are message-passing app environment where async becomes easier. They are duals after all.

And the advantages of async (or message-based) software systems are many. Too many to go into here.

I think it is not just file and cpu and it is not safe to assume that everything depends on a single box in most cases (I think that is safe when talking about an OS as this is) An example would be network calls and database connections. I think Async can be helpful anytime one component can be much slower than others and that component cannot take advantage of all the resources the running machine. Even if that component could take up all the resources you may want to have interrupts or priority as well.

The main reason asynchronous programming exists is that useful programs generally have to communicate with some part of the outside world, which does not share the CPU's clock. To a very near approximation, all device I/O is fundamentally asynchronous; synchronous I/O is either an abstraction created by a multitasking operating system, a spectacular waste of CPU time, or a careful optimization used in certain circumstances on an embedded device. Yes, you really can async your entire application; having learned to code on primitive home computers in the '80s, I generally prefer to.