To be honest, I don't understand what substantial benefit end-to-end encryption actually brings in an environment of (almost-)mandatory updates.
- If someone from Facebook/Telegram/Signal/etc wants to know what you're writing, they can just instruct their app (via update) to send them your key. For closed-source services, you'd theoretically have to decompile and audit each update to make sure they are not doing that.
- If they want to know what you have written in the past, they can instruct the app to send them the conversation log.
- If Google (or Apple or Microsoft, respectively) want to know what you're writing, they can instruct the OS to send them the data.
(Google's "Android Backup Service" for example also backs up "third party settings and data" [1]. I don't know about the details of the backup service, but this shows to me it's quite possible that your key or conversation logs might even land accidentally on some providers' servers without them having any bad intent.)
- If (three letter agency of your choice) wants to get the data, they can just force any of the above companies via NSLs to get it for them.
- If any of the US strategic partners want to get the data, they can likely make a deal with an intelligence agency.
- Lastly, if the messenger company wants to mine or sell user data, they still have a lot of stuff that cannot be encrypted for operational reasons (such as your contact list and the phone numbers of all your contacts).
That leaves to me the only group for which "overlay encryption" brings an actual benefit political activists in a country not at all affiliated with the US - or highly knowledgeable individuals who carefully control which updates they get. Both groups are important to consider but likely had ways to protect their communication before.
To actually protect communication not just from "the government" but also from the private industry, we would at least need some independent party to vet app updates.
[1] https://support.google.com/nexus/answer/2819582?hl=en