It wouldn't even take a new crop of IE exploits in the wild to make MS stock drop in price. The first two weeks would be a constant flood of blog posts detailing how crappy the code is. Trident is 17 years old, and many of us have heard how much of an unmaintainable mess the codebase has become in their attempts to implement web standards. Even then, MS would have to release it under a fully open license otherwise no one will taint themselves.
The "new thing", Spartan, is just a rebrand of IE with a new skin. It'll still be built around Trident and Chakra, so web developers will have no reason to have a different opinion of it from IE.
MS needs a new rendering engine, but they'll never use an open source one. Their only real option is to write one from scratch, which they haven't done since IE1, and that engine was replaced with Trident (which they bought) in IE4. There was IE5 for Mac, but that was pretty much a one man crusade and its rendering engine was nothing like regular IE.