I think we may be talking past each other here, what I mean is that federation must be strictly and absolutely invisible to users.

Perhaps I was wrong to say that federation is in principle ruled out, but I do think that it must be invisible to users. - Including namespacing usernames to nodes within the federation as with the @ sign in email.

I believe this is true in spite of the technical beauty of federated solutions, and it's exactly that beauty that's lured so many onto the rocks.

I don't think the namespace issue is as big of a deal as you seem to think it is. People are used to dealing with namespaces when they make phone calls, send emails, send a letter, get a license plate, or go to a website.