There is an aftermarket for F-16 planes and their spare parts.
Also, I should note that Iran (of all countries) is still flying its F-14 Tomcats, unless I’m mistaken.
The most immediate effect is ceasing updates for then AN/ALQ-131 countermeasures pod. If Russians come with a new radar or make their existing weapons behave differently, this could have considerable effect on the survivability of an F-16 in a hot situation.
There is also debate in Sweden, and polls in South Korea have turned in favour of having them.
I have not yet heard of a single case report of US-sourced military jet becoming remotely disabled.
Also note: Iran has been flying F-14 jets which the Islamic Revolution took over from the Shah’s regime.
When a country buys military aircraft, they demand extensive knowledge of the system they are buying, and demand ability to independently perform maintenance.
If they are prudent, they will review all its wireless reception and transmission capabilities, possibly on their own. There may still be a possibility for a logic bomb somewhere, but a logic bomb in flight softtware would ordinarily mean two things:
These are pretty big disincentives.
P.S. Cryptography: one can likely configure an aircraft so that it won’t accept a message through its data link system, unless the message authenticates and decrypts. Subsequently, one can change keys to no longer match a compromised ally’s keys. As a result, direct data links would no longer be possible with planes of that compromised ally. So introducing a specially crafted message into a military plane would likely be hard.
This could be an important move, since Ukraine does not have satellites of its own currently.
Ukraine has been relying on US and commercial satellites to monitor Russian activity in areas where drones cannot reach, or where they cannot communicate.
Commanded by the Trump administration, the US military recently stopped sharing intelligence data with Ukraine. Commercial data tends to have a higher latency and lower resolution, so I think Ukrainians will highly appreciate French assistance.
To my understanding, EU countries aren’t in a shortage of aircraft - their air power is enough to match Russia. However, they are in a shortage of bombs to drop and missiles to fire.
They’re also in a shortage of artillery and rocket artillery and air defense.
As for the industry that can be scaled up fastest (drones), everyone is in a shortage of them. Fortunately there is one country in Europe that’s been doing absolutely everything to scale up their production. So much that it currently out-produces the US, and maybe out-produces both the US and the EU. I’m fairly certain that this country is willing to help on the matter (it’s called Ukraine).
Note: Iran still uses F-14 Tomcats.
Possibilities for implanting a logic bomb are endless, but there are still no case reports about an US-originating plane becoming remotely disabled.
(Russians would absolutely like to listen to a message which accomplishes that, meanwhile allies would definitely want ot change encryption keys on any plane which is supposed to receive that. I also don’t think that military planes will accept unencrypted messages. And their communications subsystems are separate subsystems, which can be disabled or replaced.)
Meanwhile, in a hypothetical doomsday scenario, Danish F-35s would land (or be unloaded from a container) in China, and be greeted with cheers, red banners and golden confetti. Damage to the US would far exceed anything that Denmark could do by firing something.
Note: these are scenarios which are not supposed to happen, but dramatic loss of trust among allies can actually result in that - if country A backstabs country D, there is nothing that really prevents D from betraying A to C.