If you're monitoring real time power consumption you then need a whole extra infrastructure to communicate this info back and forth. Of course you then have to consider how you're going to keep that extra infra online in the event of power issues.
If you find yourself in the middle of a black swan event, and 15 GW have tripped offline, you have milliseconds to dump pretty much exactly 15 GW of load, otherwise more generating capacity is going to trip offline very quickly.
If you only dump 14 GW because you used historical data (which happens to be imprecise, because today's cloud cover reduced rooftop solar output), you're still going to be in trouble. A detector scheme with sensors at every substation would allow you to do just that.
I also wonder what the realtime requirement is. Data from a minute ago is fine .. except in this kind of situation, when things are changing very quickly.
The estimates we get from seasonal studies are usually close enough, especially since load shedding isn't a finesse exercise.
The situations that require load shedding usually give operators only a few minutes to react, where analyzing the issue and determining a course of action takes the lion's share. Once you're there, you want the actual action to be as simple as possible, not factor in many details.