Just as one data point, this looks great but there is no way I could use a web mail client for security reasons (as a security consultant, we use PGP a lot and need a local email provider). In the comments I see it only works with Google servers to boot. I'm not sure if this will ever be for me as well, but I'd love something like paying a small fee for a version of Thunderbird with one or more of these features patched in.
Shortwave cofounder here. We totally get that. Gmail is just the start for us. We plan to support more providers in the future and even become our own provider long-term. Read more about our company mission on our blog [1].