This article seems a little unfair to the WhatsApp founders when it suggests they should have known that any assurances Facebook made to them as part of the acquisition were obviously lies. Sure in 2018 it might be obvious to most of us that nothing Facebook says or promises can be trusted but I don't think that was necessarily as obvious when the acquisition happened and they can be forgiven for thinking they weren'…
It's not their fault as it was not their responsibility to provide a private message service for free to millions of people.
This is just how it works, if want to be principled you can't make agreements with the devil.