Not that easy
Been there and done it with OEM level resources and better documentation & familiarity than your average end user nation state would get. It's a very very specialist task, even within a specialist field (in part because it's not usually necessary) and doing it on the actual platform is asking for disaster.
And that's assuming anyone jumping in would have a clue what they were looking at, avionics isn’t based on the standard BSPs or bootloaders or schedulers or open source. They likely wouldn't even recognise how the processor was configured compared to the standard off the shelf initialisation 99.9% of people use.
It's one thing to fiddle on a rehost and have it work 'well enough' after patching, doing it onto the actual avionics will likely work just well enough to break it, or potentially just trip over some hostile anti reverse engineering features that everyone just loves to put into their special military systems that run a risk of being captured & analysed by nation-state level resources. It's not a PlayStation or a phone.
And you'd be very very brave to trust an unofficially patched aircraft software load. It takes long enough to get a proper one out the door, especially if you actually want it mostly bug free.
The Dutch can make all the claims they like but it just isn't that easy. And they haven't even got a relevant domestic OEM or supplier to help out.
As for a remote kill switch, seems deeply unlikely, too much risk. Something that nobbles people operating outside the official supply chain after a while? Quite possible, and that can be very sneaky, under the pretense of ensuring the users are operating correctly with OEM support.