We believe the Thunderbird experience is better for moderate to heavy e-mail use. It’s much easier to process incoming mail – anyone who’s had to use web mail on vacation to deal with dozens of e-mails can testify to how tedious it can be.

https://www.wired.com/2007/04/mozilla-why-desktop-e-mail-crucifies-the-browser/

Web apps are not the answer. Native apps are superior and will exist for a long long time. The day it is only web apps is the day I will quit using computers in any form.