Consider that right now to intercept end2end encrypted messages (and lets just assume facebook has that) a malicious actor has to get physical possession of your device, which implies that you have to be specifically targeted. With a backdoor malicious actors would then be able to target many other things, including for example facebook's servers, to access those messages (and all of them at once). I think simply avoiding the bad PR of "hackers downloaded everyone's messages between 2017 and now" is most of why they want this stuff end to end encrypted.
If Facebook implements a switch in the app that would send your decrypted messages back to them, then a malicious actor would have to compromise Facebook to obtain those message. Right now, a malicious actor can obtain your messages by compromising Facebook and implementing that switch themselves. From a security standpoint that seems identical to me.
The app can bundle the message telemetry with the rest of the encrypted app telemetry when it phones home and even the best security researchers would not detect it. It can also process it locally for profiling and other advertising means.
By examining only the network traffic, sure, but what about by examining the code?
That hasn't necessarily stopped people from learning something about how it works, though.
And that' not even taking into account that it might be possible to push something different to each user.