Since the newest Airport Extreme routers don't support SNMP, how can I monitor bandwidth usage on my network?
PFSense, DD-WRT/Tomato, Peplink, or some other router in front of the Apple AirPort Extreme will likely be the best solution.  Don't think there's an easier way unless you're running on a single machine for your whole network (not likely).  All the routers I mentioned support realtime traffic monitoring.  NetGear Nighthawks have this capability too.