A significant part of the economy perhaps, but 'national security threat' is a somewhat higher bar IMHO. LHR has a role of convenience, but not necessity. If JFK was shut down for a day or two and had limited operations for another week it would be inconvenient but would barely register in the national economic stats. I am on a flight heading out of Heathrow on Sunday for work travel and have booked an alternative out of Gatwick just in case. Inconvenient, but not a massive problem.

What will be telling here is how quickly things adapt to the disruption. I expect to feel more impact from the loss of power to businesses in the surrounding area that are involved in air shipment than in the flight disruptions (e.g. cold chain logistics and inventory management for just-in-time processes that warehouse near the airport.)