Can anyone explain something? Since there are so many open source chat applications, what keeps anyone from "just" exchanging a key with someone else out of band, and then modifying the client so that it uses that key to encrypt all communication? I understand that this does not scale to big groups, but surely whoever is pushing this crap must have thought about this? Or is the idea that we will have completely locked down PCs as well ala android and ios so you can't run anything unapproved?

Nothing is stopping people from doing that. It’s just more inconvenient than other available options. If I were a politician trying to remove private communication, I would first pass something that allows scanning communications, then pass something that allows e2ee messages to be scanned, then make it illegal to use non-scannable e2ee messaging. People could still do it, but now they could be punished if they’re ever caught