Why wouldn't they just patch that part of the code? I'm seriously surprised though Microsoft didn't build an adblock in IE much earlier, that would have stopped google in the tracks.
Google will begin by deprecating the webRequest block API (that's in progress). Then they will begin deprecating the code that supports it.
Then they will begin removing and deleting the "unnecessary" code. Then, at some point in the future, they may make other breaking changes to the webRequest flow that assume that requests cannot be blocked. Each time Google writes new code that assumes webRequests cannot be blocked, Microsoft must patch that code too.
With each successive change, Microsoft will be forced to make a choice:
1. Support both Google's webRequest API and the Microsoft's, at increasing cost with every merge and rebase where Google has cut off or altered another piece of functionality
2. Adopt Google's API and break ad blockers
Google has a profit motive in shrinking the ad blocking market, and therefore they have a perverse incentive to make their browser worse for users, and more expensive for any forks that want to maintain ad blocking functionality.
Merging incompatible changes seems like a win compared to doing everything themselves.
Chrome will deprecate in a release, approximately 1 year later Chrome will block updating of any extensions using the old manifest, and after approximately 18 months they will remove any extensions from the Web Store that have not updated, and approximately 2 years after the deprecation later the code to support the manifest will be removed.