The disk is almost certainly not idle, but instead the iops are consumed by read traffic.

Ideally, the kernel would know the deadline by which the data should be committed, manage accordingly, and the fsync would be simply acknowledged rather than inspiring extra IO.

This is pretty hard to work towards and the clearly specified narrow syscall interface leads to a great deal of difficulty in sharing this information with the kernel IO scheduling -- the way it is shared would naturally depend on the details of the kernel implementation, which would imply a tighter link between application and kernel version than is normally desirable.