That's a pretty big difference.
Also, a lot of computer use is via mobile these days and I doubt too many people are using the web interface on mobile for gmail.
And point about conversations moving to post-email protocols, but email is certainly still up there with HTTP as a bedrock standard that everyone eventually touches.
As for their move to full mobile app, I think it's a bit of a different calculation when you happen to own the OS that powers ~75% of all mobile phones . ;)
Suffice to say, I don't think Google has the same troubles as other developers. (Exception to security policy, for my first party app? Sure!)