For your last point, there are many mice with 2+ side buttons, which are mapped as forward and back by default and can typically be remapped, it works in pretty much any application that has a "back" option.
Don't feel it is the same, because desktop applications don't work the same. And when you are on the "last screen" and press back on a phone you get to the home-screen. So in most cases a single tap or double-tap takes me to the home-screen.
So it is both for going back and "quitting" applications.
On the desktop it wouldn't even work in the browser, because I work by tabbing new pages and closing tabs rather than navigating back.
So to be fair, I don't want it om my desktop. Because I really don't want my applications to mimic the behavior they do on phones, that would be horrible. But the end result is really nice for the weird usecase of running a phone inside a desktop.
