Fast Company claims that the main reason for moving users so aggressively to a separate messenger app lies in their move to facilitate user-to-user payments. And, of course, user-to-business payment is not far away from there. They even show screenshots as proof, but anyone could have just made those up.
Still - it could make sense. With Apple pay at our doorsteps and SMS phasing out, which in some developing countries has essentially been an alternative currency system, this could be an interesting strategy. When you think about 'how to monetize a reach of over a billion people with something else than advertising', you quickly get to ideas that are not typical B2C models but infrastructure - high scalability, low risk, platform oriented, generating a network of parties that benefit from it. I think it's a legitimate theory.