Let's say MS rolled V8 and Webkit into IE. They still get to build their own UI/ux and get to set the default search engine to Bing. They start contributing to webkit and everything gets faster.
There are plenty of things to implement in a browser beyond the rendering engine, so why create a whole new one that web devs need to support?
Removing the operating system tie-in alone would be worth it. Unfortunately MS still lives in a world where people should pay upfront for their web browser.
See https://wiki.mozilla.org/Platform/Layout/CSS_Compatibility#q... for instance
Remember when IE was the only browser worth worrying about? Wasn't development so easy because IE was the only browser you had to support, and IE was miles ahead of its competitors like netscape navigator? The result of that was stagnation, and I would rather not go back there.