I think you are looking at it from the wrong angle. The WM is just a component of a complete desktop environment. Instead of making a Tiling WM that comes with the kitchen sink, we should make a Tiling WM that simply replaces the WM on the pre-existing, and fully featured, desktop environment you already have. Technically speaking, this is not challenging. For example, to make Awesome the defualt window manager in gnome, one need only run:

  "gconftool-2 --type string --set /desktop/gnome/session/required_components/windowmanager awesome"

Obviously, this is a large barrier for a casual user wanting to try a tiling window manager. However, if WM installer changed that setting instead of just creating its own desktop environment, then we would not be having this problem.