That is absolutely not true. We regularly export weapons systems with embedded capabilities that are not accessible to the folks we sell them to regardless of their ability to "decompile" the software and firmware they contain. We've been doing it for decades and we are very comfortable with the results. That is all I'm going to say on that subject.
"Even stealth fighters are facing obsoletion now that Israel has fielded video-targeting anti-aircraft missiles"
That is not new technology. It also assumes you know the stealth aircraft is there in the first place. Which is hard to do without some form of radar.
Let me assure you that 1st world nations such as the UK and Israel and the U.S. can access and decompile any and all software and firmware in hardware that they possess.
You would have to download encrypted software on-the-fly on an as-needed basis to beat First World, state-supported hackers who have physical possession of said software/firmware/hardware.