On one side, you download 70MB of thunderbird once and update it every once in a while without losing access to your emails while it's updating. On the other side, you have to download 25 MB of Gmail more frequently because cache is invalidated at a higher rate than thunderbird updates and you have to wait for your browser to download most of the page before you gain access to your emails.

Don't get me wrong, I use webmail all the time with protonmail, Gmail, outlook and purelymail, because it abstracts installation and updates, but your argument doesn't seem to be valid.