And it would instantly break every single intranet built on MS technologies.
They could bake in Trident (IE’s rendering engine) and allow intranet administrators /IT to declare intranets render in IE7/8/9 mode. They already do something similar with their browsers, and IETab for Firefox essentially does this.
(Edit: This is obvious to industry observers and has been “requested” by frustrated devs for years. But Microsoft won’t do it. Pride, probably, is the biggest reason.)