There are many practical reasons that people and organisations would rather be in control of their communications, so XMPP (or something, the protocol doesn't matter) is here to stay. The challenge is making that open network worth the big players' time, they currently don't seem to think it is.
To deal with an onslaught of spam, Google disabled incoming subscription requests for a short time. During this time, all other XMPP federation features continued to work.
Half of these things (e.g. carbon copies) are spec'd out but barely supported.
Would it not have been possible to offer a XMPP MUC as a text-based interface to hangouts, to allow ‘legacy’ users to still be connected?
The X in XMPP is for extensible. I guess the point is Skype doesn't support XMPP or federation, and Microsoft is moving towards Skype and away from their SIP/SIMPLE chat services. Likewise, Apple's promise of an open standard for interoperating with Facetime turned out to be empty.
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Edit: Found this: http://www.joecieplinski.com/blog/2013/03/30/hey-apple-where... http://www.imore.com/apple-fined-368-million-facetime-patent... http://www.techradar.com/news/world-of-tech/virnetx-slaps-ap...
And as you say - there's a bunch of stuff to support if you go the traditional route and try to bolt on more crap to MUC or <message> stanzas. We're considering just building hangouts on top of "ephemeral" buddycloud channels that just disappear when the last recipient leaves them.
XMPP also has a spec for server-side history storage, that allows for syncing to clients. I don’t know how well it is supported in reality, but understand that there are at least ejabberd modules for the server-side part and a plugin for Pidgin to view this stored history. Not perfect, yet.
I guess it is a long way till we get there, though :|