Not great examples. HTTP has evolved several times to address changing needs: HTTP 0.9 (1991), HTTP 1.0 (1996), HTTP 1.1 (1997), HTTP 1.1 again (2007) and HTTP 2.0 (targeted for 2014). So did SSH: SSH-1 (1995) SSH-2 (2006).
These evolutions were possible because there were multiple major players that could come together (with multiple implementations) to settle on the new standard.
> but it sounds like XMPP at least could have simply been extended if there were features Google needed for Hangouts
In an alternate future where Google did that, people would have cried "embrace, extend, extinguish." Especially if any of their extensions were anti-spam related, people would have criticized them for being exclusionary.
For example, look at all the flak Google has gotten for SPDY. Evolving XMPP would have probably led to the same criticism.
And XMPP still wouldn't have provided interop with any of the major IM networks, because none of them were implementing it.
You truly can't please everybody, even if you really believe in something.
Extensibility is a built in part of the XMPP standard, and Google have already made several extensions related to Google Talk etc.
Using XMPP provides a much greater possibility of interoperability, as several other major "modern" IM networks use their own non-federated XMPP network. WhatsApp, Facebook Chat, etc. It's much more likely that two non-federated XMPP networks will become federated, than two binary incompatible networks both becoming binary compatible AND supporting federation.
I'm fine with Google dropping their support for open standards - their services, their rules, and if they don't care about users like me who prefer the open standards, then it's not my place to whine about it when I can instead seek a competitor that does adhere to those standards. However, I'm not fine with such dropping of standards being masqueraded under some lame excuse like "the open standards are too old". 1998 and 1986 are young compared to the majority of the standards which form the backbone of the Internet and World Wide Web themselves. If you're going to try and rationalize Google's actions beyond them wanting to make money via control over their stack, then at least try and find some actual technical reason why XMPP and IMAP are insufficient for their purposes.
(I'll concede that IMAP isn't exactly pretty, but it sounds like XMPP at least could have simply been extended if there were features Google needed for Hangouts).