"Unless a push is built-in, some server admins may not update, leaving features unavailable depending on which server you've logged into. With the constant innovation in web apps and user demand for a more feature-rich experience, that's an uphill battle for a distributed system."

That's backwards. In a distributed system there's competition between nodes because it's relatively easy for a user to move from one node to another. In a centralized system the competition is with separate systems, and since it's harder to move away an entrenched central system doesn't need to do much to survive.

Individual nodes also don't need to be as conservative about new features as massive centralized systems do.

There is a point where inertia sets in and new features that require widespread adoption become impossible (this is where email and IRC are now), but that takes years.