Thunderbird is great. I’ve been using it a long time, too, and feature-wise, I think Thunderbird is way ahead. I just find Mailspring a bit more... pleasant to use? And I think it’ll be an easier switch for people coming off Gmail Web, which is why I’m recommending it.
(Also... Thunderbird is not that different from an Electron app itself. It uses Gecko instead of Blink, and has a few bits of XUL here and there, but the core premise is the same. Though it doesn’t use React at least!)
> I would go further and ask what do electron apps offer in the way of improvements that WebApps don't.
If there was a way to run Mailspring on the web, I’d switch in a minute! I’ve tried to port it one time, but it got a bit tricky.
Mailspring uses a native module for syncing the mail, which would have to run either on a server somewhere or in a web worker. I think the web worker makes more sense nowadays, and would make it an offline-first app, but there’s a catch: how do you connect to IMAP/SMTP from a web browser?
Running the sync engine on a server is possible, but you have to have a server. It could make sense, though, if you’re running your own email server and want a killer webmail app to go with it.