The proposal is to be able to run io module operations in two modes: the regular one, and one where performing actual I/O is forbidden – so if they go down the stack and can fulfill the operation from some in-memory buffer, great, they do that, and if not, they raise an error.

It turns out that this is actually the right primitive to enable async disk access. That's the only target use case, and there's no IO loop involved. If you wanted to keep async disk access separate from the io module, then what we'd have to do is to create a fork of all the code in the io module, and add this feature to it. Which doesn't seem like a good design :-).