A cynical person could also make a counter-strike: display some warning popup in the regular XMPP clients each time somebody tries to start a new chat with a user @gmail.com and doesn't receive a response in 2 minutes. "Warning: the person you have tried to contact uses gmail.com. It is possible that their account has been migrated to the Hangouts platform and that Google has silently discarded chats you send them. Please click here for a help page you can email to your contact to fix their account."
Well, this logic can be refined. The broken resources start with "Messaging".
Not sure why nobody at Google cares, but they didn't bother with activating TLS.
