Not just the UI side. The underlying protocol was also pointlessly over-engineered. SMTP is still around because it is simple. You can get a basic SMTP client running in an hour or two. You can get a basic server running in a similar amount of time, at least in a higher level language. That matters, not because anyone will have anything usable in that time, but because it lets you start to experiment very, very easily.

Wave suffered from a UI that was a mess layered on top of a protocol that made it really hard for people to get started on experimenting with alternative frontends, and without a user-base giving people a reason to persist figuring out how to interoperate with it. Had the protocol been simpler, the UI mess might not have mattered so much - people might have come up with their own ideas.