Your first point is against their terms of use. So, that's preventing them to do something like that. But sure, if you think that Facebook will break the law here, switch.
For your second point, how would Google ever get my device backup? I guess if you are using Android and worried about your privacy, what you should switch is not your messenger app, but your OS.
agreed i sound a bit pessimistic here, but companies regularly show that the only thing that matters is the bottom line, so i stand by the assertation that breaking the law/eula is not really a question of ethics.
second point; you and i may not be using android and google backup, a vast majority does.
For your second point, how would Google ever get my device backup? I guess if you are using Android and worried about your privacy, what you should switch is not your messenger app, but your OS.