I mostly agree... I do think it's more of an "if you build it" scenario. Thunderbird isn't big as a mail client, because there isn't the level of integration with either existing providers and/or a self-hosted option.

o365/exchange, gmail/gdocs/gcalendar and fastmail are probably the big three providers that should have seamless integration at this point. A SaaS option from Moz would be nice, as would a self-hosted option. I bring these up in line with Moz's efforts to get VPN subscribers, they could likewise do groupware.