They don't need to. They just need to stop it becoming a full competitor.
Facebook's asset is the social network, your contacts. WhatsApp was a threat because they were getting a social network to match. That could then have been acquired (by Google) or developed a more social platform on top of it. By owning it Facebook can keep WhatsApp limited to a messaging platform only and remove the risk and limit the damage to Facebook itself. Next step moves to encourage WhatsApp users onto Facebook for group messaging type services.
That's kind of what I was fearing. Would love to see an official response from Moxie on the meta and contact data handling of WhatsApp's implementation.
This is presumably why Facebook bought it in the first place, though. They don't need to see anyone's message content for this. They get all the info they need the first time you use it; confirming phone number 123-456-7890 belongs to person@domain.com, so they can tie together your facebook usage with whatever customer surveys, browsing etc you've done. I don't see this as remotely altered by the recent encryption work, other than the latter will entice more people to use the app.
And I think that's really the messaging that needs to have some PR light shed upon. Facebook has access to the meta and contact data (confirmation, Moxie?). For all the hoopla over WhatsApp E2E crypto, users shouldn't forget that they may be handing over phone number, contact data and other meta to the largest ad-tech platform with a real name policy.
I'm asking because I could imagine that Whatsapp might get banned in some countries soon (as recently happened in Brazil) and thus, lose market share.