I think you're wrong, just because it'd be easy for Wave provider's to legacy-support email/SMTP: just dump incoming emails into your Wave Inbox, flag it, so if you answer it, it is sent back over SMTP. Or let Wave users append to it like a regular wave message, but the user has the option to send it back to the originator as a HTML email. Sounds doable.

More interestingly, given the fact that developers can write apps against the API, this may have a non-trivial intersection with social networking, meaning this could be a competitor to sites like Facebook, Myspace, etc.