Such a system is not used because of the differences in ownership between metro, regional and remote stations. The current system was adopted to prevent overlap for the commercial networks and their affiliates.
In time, they may adopt national numbers if the metro networks purchase any of their affiliates, but we’re stuck with what we’ve got for now.
The 350-399 range is used for overlapping signals for ABC and SBS, so if they just expand that to the whole 3xx range (310s Ten, 320s ABC, 330s SBS, 370s Seven, 390s Nine). I personally would save the 1xx range for HD/SD simulcasts.