One of the Biggest Privacy Wins in Recent Memory Has Been Reversed

You make it sound new, but the article makes it sound like it's been an issue since the 70's, except maybe for narrow instances in the last year with the previous ruling. I've been aware of and uncomfortable about digital applications of the "third party doctrine" for a few years already -- does this ruling open that door wider?

end-to-end encryption is nice, but since the government can force companies to secretly add backdoors, it is pretty much game over for privacy?

It's not clear how cell phone carriers could use end-to-end encryption to protect the particular kind of data at issue here; from the point of view of the GSM protocol, the carriers are the other endpoint. It's like the way your ISP couldn't use encryption to blind itself to whether or not you're currently using your home Internet connection: if packets are being routed over that interface, you're using it.

I think this situation is horrible, but we don't, in this particular case, have a ready-to-deploy encryption remedy that carriers could simply choose to enable. A lot of the problem is found in the architecture of the mobile network itself.

For the larger picture, a challenge is the prospect of much larger numbers of users yelling back at the companies who do adopt end-to-end encryption, because the companies become unable to reset the customers' forgotten cryptographic passphrases. So you can expect yelling on both sides. (I heard that one company that specifically advertises how it can't read or access your data already gets a considerable number of support requests from users with forgotten passphrases who, in the moment, don't believe that's even possible, and the company must be lying because it doesn't want to help them.)