I think the single graph idea is impractical in any large organization. Every centralized model deteriorates over time due to many reasons: time pressure, carelessness and bigly varying competence levels in federated teams. A couple of years down the line everyone will hate it, and it'll be too big to discard.
Independent team structures and microservices based architectures are acknowledging that such monoliths are impossible to build and maintain.
The point of federating your schemas across different services is that you can achieve the single graph that represents the organization without having the separate parts step on teach other. It doesn't discard microservices, it embraces them.
Independent team structures and microservices based architectures are acknowledging that such monoliths are impossible to build and maintain.