Maybe the key to improving incentive is to bind the acts of participating in and supporting a decentralized service so that simply using it helps construct the network itself, i.e. every client is itself a federated server. The tradeoff is that the service has to be robust enough to support a certain percentage of clients being offline at any moment, and it certainly takes more effort to construct a client that includes what would normally be delegated to a server.

(I'm sure someone's already thought of/worked with this idea before, though.)