That is awesome! I actually toyed with the same idea a couple of months ago and started a prototype, but in the end it was too much work for a single developer (having to implement IMAP/POP backends, etc. in addition to the UI).
Using web technologies for desktop apps is not ideal, but at the same time it's probably the easiest way these days to build cross-platform apps because of the lack of a good cross-platform UI toolkit.
A big benefit I see is that it will make it very hackable as a lot of developers are familiar with web technologies. That should be a good way to get a lot of contributors to implement features and innovate (See for example the community / contributions around Visual Studio Code).
