Canada

There are no F-35 parts only sourced in the US. Every country in the F-35 program has the ability to manufacture parts. The only thing the US controls is the firmware. The US can't lock us out of buying or building replacement components, they can only lock us out of getting newer firmware updates (eg, we can still use the firmware we have, just can't get anything past the point where they decide to cut us off).

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.

Have you noticed the very large, very aggressive expansionist autocracy on the other side of the arctic circle from us? Do you consider that maybe they might also be a potential threat?

Having a plane that massively outclasses everything in Russia's fleet is not a bad idea, no matter what is happening to the south of us. A mixed fleet hedges our bets, giving us responses to both of the biggest threats we're facing right now.

It's a waste of money if your primary goal is defending against the US, yes. Which is another reason why all the handwringing about how the US could potentially compromise the F-35 is really kind of silly. Truth is, if they wanted to attack us, they wouldn't need any kind of magical "Kill switch", they could just rely on old fashioned air superiority.

I'm still a huge advocate for signing a deal for the Gripen anyway (as stated above, I like the mixed fleet idea the best), because it gives us a lot of economic advantages, allows us to become an exporter of fighter aircraft to the rest of NATO and numerous other countries, and starts building up domestic air production that can then be converted to putting out Saab's new 6th Gen fighter when that's finally ready for prime time.

But if we consider our biggest realistic air threat to be Russia - Note that I said "biggest", not "only", trust me I am in no way dismissing the threat of a potential US invasion (in such an eventuality I'm on the list of people who die or get arrested in the first 8 hours, I do not fucking need to hear it) - then yes we absolutely do need the ability to patrol our own skies, especially if we end up in what is still the more likely geopolitical scenario right now, which is that the US abandons any notion of defence obligations to their allies as they grow increasingly isolationist.

A contingent of, say, 40 F-35s doesn't sound like a lot, but when those 40 planes have the ability to shoot down anything Russia can field without even being seen, that actually makes a massive difference in their ability to operate in our airspace. Right now Russia barely has 4th gen craft. The Felon is a lie wrapped up in a cool video package; it's radar signature is fucking massive, and we've seen from Ukraine that Russia's own radar capabilities are mediocre at best. And they have, optimistically, 6 of them. Anything else they have is so far outclassed by the F-35 that it would be like Challenger 2s going up against T-55s in Iraq. That's a hell of a lot of deterrence for comparatively little cost really.

Its a long, long border, mostly undefended in the west. A lot of infrastructure undefended, and a very brittle supply chain when it comes to civilian logistics and power. A handful of fighter jets would be a nightmare for rail lines and oil depots.