Seems like a userspace write queue would solve the write-delay problem. On the other hand, it's very dangerous to report success to the user before the changes

Seems like a userspace write queue would solve the write-delay problem. On the other hand, it's very dangerous to report success to the user before the changes actually hit the disk, because they might make some other change to the world that depends on the contents of that file being on disk, leading to an inconsistent state.