I think the common way to read "you will own nothing and you will be happy" is not as a prediction/statement, but a command about how people should feel.

For example, if I told said "you will live naked in a ditch and you will be happy", the "you will be happy" part reads like a dictate about how you should feel about the "living in a ditch" situation. The obvious disconnect between telling people they will be paying monthly dues to a bunch of rent-seekers and that they will somehow feel happy about this situation even though it sounds, frankly, awful has the same effect.