A. XMPP is the major leap since HTTP. Wave is just some bloated junk that abuses it.
B. XMPP is not the intermediary network that allows anyone in the world to talk to each other, the Internet is. XMPP is just the model that makes it practical.
Just as the main job of the HTTP protocol is to define client-server communication. The main job of the XMPP protocol is to define client-gateway-gateway-client communication.
Wave is just a bloated, locked-in protocol for asynchronous user group communication. Since it targets users directly, and not communication in general, it will ultimately lose to customer-driven social networks. Meanwhile, you can build stuff on XMPP that's a lot more awesome, because like all the successful protocols before it (IP, DNS, FTP, HTTP, TCP, E-mail, Web Services), it doesn't really do that much. You decide what it does.