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.
I would love for someone to do this, so we could have a nice, self-hosted store for our vital data but still access that data from all our devices, over VPN, etc.
The problem, as always with these things, is who that someone would be. While many HN readers might appreciate the potential advantages, I fear the target market for such a product is significantly smaller than for Thunderbird, which for better or worse is itself aiming at a relatively small minority of users in the era of Google Mail and friends.
For anything it's worth, I'd contribute financially to such a project personally and so would my businesses, since it would be a significant benefit in each case.