This isn't always true. The phone system remains well federated, even within a single country. Email is likewise hanging on, although gmail is very popular. IRC is an interesting case, where "networks" have federated servers that load balance completely transparently.
In reality, users tend to concentrate in one or two servers. And then federated is about as good as centralized.
Reminder of what happened with XMPP: It got embraced, extended and extinguished... by Google.
We should be moving towards fully distributed architectures. Only when there are no servers is this problem really gone.