In any system that uses mmap or swap it's a meaningless distinction anyway (which is obviously nearly all of them outside of embedded RTOS). Accessing even something like the stack can trigger implicit/automatic IO of arbitrary complexity, so the concept of a function that doesn't do IO is meaningless to begin with. Async/await isn't justified by any kind of interesting type theory, it exists to work around limitations in runtimes and language designs.
This argument is dull, nothing in programming can do anything perfectly: is catching exception is useless because the program can be killed by the OS? Is static typing pointless because cosmic rays can make your data ill-formed anyway?
All abstractions are leaky, and the OS and hardware's behaviors are always going to surface in ways that you cannot model in your programming language, no matter how low-level you want to go (asm itself is a poor abstraction on top of how the CPU actually works), but that doesn't make them useless.
Async/await is a way to communicate intent between developers on the same project, and between a dependency and its dependent. Exactly like static types, error as return values and non-nullable types. And like all of them while it doesn't prevent all bugs, it definitely helps.
The fact that it makes straightforward to implement the best possible performance is just the cherry on top.
