However, I don't see why you think it would be impossible for most users to set up what I'm describing. If they can install Thunderbird and configure it to retrieve mail from their ISP or GMail or whatever, they could install a two-part system and configure the store part to do the same using exactly the same information.
In particular, just installing both parts on a single PC would work fine and provide equivalent functionality to current standalone mail clients like Thunderbird. You just have the flexibility to connect other devices to the same store and safely access your mail from those as well, or to install the store part on some other place on the network like an office server or maybe a NAS at home if you want to.