As much as I've complained about email and how its services are structured, when I think about the idea of replacing it, I can't imagine why?
Its not that I like the way email is, but I have already taken the time to figure out every odd little idiosyncrasy regarding it that its finally started to work pretty well for me.
Even if the smart folks behind Google come up with something far superior, I'm not sure I'll switch, because I don't really want to go through the experience I have had with email all over again.
So for me, Google's Wave product/platform/protocol won't replace email.