That's true, but for smaller screens and only one or two apps per screen (in split screen view). I think the goal of multi-window might be to support larger display extensions (e.g. plugging a 24" display into a phone)?
Thing is that i think most are already running everything maximized (unless they are OSX users). The only things left floating are calculator and IM clients. This because the content will more often than not require the use of the majority of the screen, and they switch windows via a taskbar or similar.
BTW, i seem to recall that the original GUI design at Xerox had things tiled upon opening (they could be moved afterwards though).
Maybe people run most things maximized because window management on Windows is so limited? With 8.1 and now 10 you can arrange up to four windows on the screen without using the mouse, but even that is too basic to be useful compared to tiling window managers.
I think Google could actually pull this off by "forcing" users to use a tiling concept (like a couple of different layouts to choose from and letting you launch/drag apps into the screen areas).
I run most things maximised because, even on a Macbook retina display, I only get 1280x800 effective pixels, and that's just not enough to run more than one window at once. If I had a 5K desktop, it might be a different story. The only things I ever tile are side-by-side Finder / Terminal windows. Never see my wallpaper.
But they already have! Anyone who's been using a smartphone this decade is used to it by now.