But if we assume a government is determined to do this, can't they find other ways to do it? If you were using Google Photos with your Purism phone, it doesn't matter what you do on your device. And you can say "well, I wouldn't use that," but maybe your ISP is convinced (or required) to do packet inspection. And then you can say, "But I'm using encryption," and the government mandates that they have a back door into all encrypted traffic that goes through their borders.
And I would submit that if we really assume a government is going to extreme lengths, then they'll make it as hard as possible to use an open phone in the first place. They'll make custom ROMs illegal. They'll go after people hosting it. They'll mandate that the phones comply with some kind of decryption standard to connect to cellular data networks. If we assume an authoritarian government bound and determined to spy on you, the assumption that we can be saved by just applying enough open source just seems pretty shaky to me.
So, I certainly don't think that a purely technological solution is enough, in the long run. This is a policy issue. I think hackers and engineers really, really want to believe that math trumps policy, but it doesn't. By all means, let's fight for strong encryption -- but let's also fight for government policy that supports it, rather than assuming encryption and open source is a guarantee we can circumvent bad policy.