The hypothesized "video chat" protocol extension to XMPP already exists and it's called Jingle. FOSS client support isn't great, but it is progressing. So if this service isn't open to FOSS clients, that's more an issue of strategy than resources. (And indeed, the decision might change in future and be attributed to improved FOSS client performance.) If the service is indeed closed, however, they probably ought to update this page:
https://developers.google.com/talk/open_communications