For years, Intel and AMD processors have supported patching using microcode updates. Until we know the embargo is lifted and we know the full extent of this vulnerability, we won't know if that would be possible.
Or maybe it could be that Intel privately disclosed already that no backport will be done to firmware of older CPUs, in which case the kernel update is the stopgap for newer generation and the solution for older generations.
With a future os-specific microcode patch a config option would make more sense. They just need to find a 2nd C3 register to seperate user from kernel space or do the permission check before prefetches. Like AMD does. On Linux this check would be cheap, on Windows NT not.