What are the aerodynamic abilities of a drone swarm? That is, to what altitude can a swarm reach, to interdict an incoming aircraft? How fast? My impression of current drones is that they are fairly small, less than 2 metres diameter, which implies pretty tight limits on range, altitude, speed, and time-on-station. So even if you have thousands of them, a conventional bomber can come over at an altitude the swarm can't reach -- or a fleet of bombers can hang off the border for hours, probing at random, until the drones run out of juice -- or a fighter can make a fast pass, dodging the much slower-climbing swarm, and drop chaff or smoke or an EMP weapon to confuse the swarm.

Bottom line, I seriously doubt drones could permanently block access to a national airspace.