Does it really matter? Do they need a browser at all? Browsers are all pretty much interoperable now. What strategic advantage is there in developing and maintaining their own browser? Nobody pays for browsers. They might pay for content or apps delivered in the browsers. That's what Microsoft should worry about. They've lost their browser monopoly, move on.
Note that if other browsers themselves adopt new Windows APIs in competitive response, that actually still benefits Microsoft because ultimately they don't care about the browser for its own sake, they care about increasing Windows API and feature usage across all browsers. But in practice competing browsers tend to be late adopters of new Windows stuff (e.g., even now nothing but IE supports DComp/DManip and Chrome is just very recently getting around to improving its Windows high-DPI and text rendering support), hence Microsoft funds IE.