Imagine it this way..
You have a locked room with a special lock and key. The door and room belong to you, but the lock and matching key(s) are handled by a "special Company". You want to change companies who handle your lock and keys. But the new company has a different shape lock, and to use it means altering your door to fit it.
The room is the Dyna. The door is the login screen to the dyna. They lock is GameSpy's login server, and the keys are the user email for logging in.
Now, say we want to change from GameSpy to Steam (just an example....) the 'door' will have to be altered to 'mount' a different locking mechanism. In programming this is called an API, Application Programmer Interface. This means that Interplay, GameSpy, and Steam provide an API (which is programming code) that we insert into SFC and alter SFC to use it. Thus SFC (the door) does not handle the locking itself, but talks to a API that does, weather it is GameSpy or Interplay, etc...
The above is 'easy', because, the 3rd party provides the server and the API for SFC to use to connect to their server. I say 'easy' because each 'lock' may have a different way of authenticating users, which means it might mean other changes on the SFC side to properly use the new 'lock'.
However the above assumes there is a replacement authentication server to use. What if we had to create our own lock from scratch, lets say, because the door was so old we had to alter it as little as possible. This means creating our own authentication server, databases, and API. This is so hard it almost certainly would never happen.
My dream*
I think it would be cool of somehow we got the Dynaverse to authenticate off of the Dynaverse.net forum. It contains it own authentication already (forum login), and would remove dependency on 3rd parties forever. But this means creating a new API and testing it, where-as things like Steam provide fully tested API's for this sort of thing already. Sort of like having to make half the lock (use Dyna forum, just create new API) vs a fully supported lock premade (Steam).