I'd assume Apple and Google would seriously consider relocating if they were so ordered, even if they ultimately decided not to; but let's say for the sake of argument that they decide they have to supply the unencrypted content of messages of apps they didn't write running on operating systems they did write.
If I was an engineer tasked with doing that, I would refuse, even if it meant leaving.
But if I wasn't that kind of person, I'd have the OS take screenshots whenever a certain app was in the foreground.
