Now that's a completely valid concern; being cut off from firmware updates would seriously degrade the capability of that part of our fleet over a long enough time horizon, which is one of the reasons why we're considering taking on a large order of Gripens as well. But it's not a "Stop your planes from flying" scenario. Even without the latest firmware we'd still be able to put an F-35 in the air and have it engage and destroy targets.
Also if the US refused to provide firmware their f-35 fleet would eventually become completely grounded and useless as a large amount of the f-35s parts are made across the world.