The case with XMPP is that Google Talk introduced addons and intricacies that were unique to them. So they could federate with you in full with additional bells and whistles while you were stuck in an eternal catch-up. They presented a better alternative regardless of the eventual defederation. Even if we have some viral clauses as in GPL in open-source software that ensures protocol compatible software to be compliant, we can only do that to a certain extent plus enforcement is always an issue. Who are going to spend the vast sum of money in court to defend the "federation"?

This aside, enforcing federation alone does not ensure decentralization. These zero-marginal-cost fixed-cost-intensive businesses of the internet has a tendency to centralize as serving one more seat costs no penny plus one more seat diluates the fixed cost altogether.