Look several years into the future and what you could have there is electron being faster/lighter.

Being web-based gives you a standard way for fetching and displaying content. It's just a matter of time that the performance improves. Especially when many people are experimenting with it.

> maybe one day we can't even reach for a native email client because it will be unmaintained in favour of the hackable html webstack email client.

It's definitely possible, once the browser becomes the new OS.