Gmail is used by what, 1.8 billion people? And e-mail isn't some small take-it-or-leave-it feature. Even disregarding the mail itself, e-mail is central as a form of online ID for accounts.
If Google tried to kill Gmail there would be an uproar of incredible proportions.
Realistically, their best bet for getting rid of it would be to gradually abandon it, making it slower, less reliable, taking away features, etc over the course of a decade or two. But even then, they'd likely need to wait another ~40 years for generational effects to kick in and most people to get off of it.