Yeah, I wouldn't be too confident in Facebook's implementation, and I certainly don't believe that their interests are aligned with their users'.
That said, it seems like we're reaching a turning point for big tech, where having access to private user data becomes more of a liability than an asset. Having access to the data means that they will be required by law to provide that data to governments in various circumstances. They might have other legal obligations in how they handle, store, and process that data. All of this comes with costs in terms of person-hours and infrastructure. Google specifically cited this is a reason they are moving Android location history on-device; they don't want to deal with law enforcement constantly asking them to spy on people. It's not because they give a shit about user privacy; it's because they're tired of providing law enforcement with free labor.
I suspect it also helps them comply with some of the recent privacy protection laws in the EU, though I'm not 100% sure on that. Again, this is a liability issue for them, not a user-privacy issue.
Also, how much valuable information were they getting from private messages in the first place? Considering how much people willingly put out in the open, and how much can be inferred simply by the metadata they still have access to (e.g. the social graph), it seems likely that the actual message data was largely redundant or superfluous. Facebook is certainly in position to measure this objectively.
The social graph is powerful, and if you really care about privacy, you need to worry about it. If you're a journalist, whistleblower, or political dissident, you absolutely do not want Facebook (and by extension governments) to know who you talk you or when. It doesn't matter if they don't know what you're saying; the association alone is enough to blow your cover.
The metadata problem is common to a lot of platforms. Even Signal cannot use E2EE for metadata; they need to know who you're communicating with in order to deliver your messages to them. Signal doesn't retain that metadata, but ultimately you need to take their word on that.