IE will remain for a while in part because to many enterprise environments which rely on IE, trust me I am with you all on the death of IE as at my work setting compatibility mode for specific websites and many of the state sites require different versions of IE and will only use IE. Until spartan becomes a standard for websites IE shall remain.
Lets just hope they do not build Spartan the same way as IE, hopefully they take more of a Chrome or Firefox approach so with each release to not break backwards compatibility.