The road of tech is littered with the bodies of email replacements.
Some of them went on to thrive, but not as email replacements: Slack for one.
Just like with Wave.
This one has the benefit of not being owned/backed by a company that must be one of the biggest quitters of all time ;-)
Most hard problems are not solved simply by creating a technically superior product, they are often unsolvable due to mundane factors like user adoption, distribution, financial issues, server issues, etc.