From MS's perspective (again: I work here, but my opinions my own blah blah) the problem is this. People use computers to access the Internet. MS can't just be the OS and the browser used to access the Internet to maintain its lofty position as a field leader -- if MS's job is to supply the Internet as a service to people on MS devices, then it is mission-critical that it also be the landing page of the Internet. If MS gives up Bing, it might as well give up all consumer investments IMHO.
It is (IMHO) more important that Bing exists and is mostly functional than it is that Bing is equal or better to Google in every way.
Of course it is a huge priority to make Bing a viable threat in its own right, but what I'm saying is that this is not the only consideration.
The correct way, I believe, is to compete on non-consumption and capture those users, therefore you start small and execute well and scale up as users grow. You avoid the comparison to Google and can sneak in the back door without the crazy upfront costs. Imagine if microsoft focused on Knowledge graph before google did? It would be very interesting times, but every google competitor just competes head on. At the end of the day its about getting users and they don't care about the slight change in your algo, only in the way they feel when using your product i.e frustration, delight, anger, surprise, etc!