[1] - http://www.computerworld.com/article/2509604/data-privacy/mi...
All you can do for now is to connect to Google Hangouts with an XMPP client, which doesn't mean much, doesn't work so well and given that Hangouts is explicit in its lack of support for XMPP, we don't know for how long this will last.
Oh well, it's Google's loss, we've moved to Slack where I work instead.
Anyhow, the only real reason to stay separate would be to compete with each other, with the slim chance to push the competition out of the market (at every services' users expense), and I agree how this has come to be a sad reality
Ironically, in May 2013 at Google I/O it was Larry Page that was expressing his dissatisfaction with the state of the industry for the lack of interoperability with messaging, lambasting Microsoft. But then just after Microsoft announced their support of XMPP in Outlook.com, Google announces Hangouts.
I guess that was provided as an excuse for dropping XMPP support. Well, welcome to the club Google, now you're part of the problem.
I wonder if Google is truly phasing it out, or if it's just an issue with the service I'm using.