I think that would just be illegal, although I am not certain... maybe it's not

What I'd be more worried about personally is metadata. Sure, they might not know what you sent, but they know who you sent it to and when. The data is generally just gonna be "Oh, this person texts their mum every morning", but Meta already provided message contents in an abortion case, so what if someone is accused of having an abortion (the fact that you can be "accused" of that now in the US is still fucked up imo, but that's besides the point) and then Meta provides info that this teenager sent WhatsApp messages to a medical professional who can perform abortions. That would obviously not work as well as the contents themselves, but it does have value to the legal case.

In the end none of us have anything to hide... until we suddenly do

I know this wasn't argued here, but I'd like to make it clear anyways: You don't have to deal drugs or be a hired killer to want privacy. There are a bunch of reasons you could get in trouble with the government which fall into morally ambiguous areas. And sometimes we just don't want our entire life being analyzed to have an algorithm decide what advertisement is the most effective in getting us to click on it.