No, closed ones are a betrayal. If someone doesn't upgrade, you can easily exclude them from communication and even build such logic in the protocol itself. It will pressure them to be up to date. I don't see an issue here.
You can if you care about security. That's exactly how the very same Google Talk was cut off from the rest of the XMPP universe, even before they decided to shut it down (they refused to support server to server encryption).
