You've not understood the situation. Locking them out of the Google Play store doesn't prevent them starting their own app store and installing that as the default app store on their Android phones. It'd then be up to each app developer whether to sell their apps through that new store. For the average developer, why not? It's minimal extra effort for extra sales. And it would make sense for Microsoft to do it; after all, the most-often-cited problem with Windows Phone is that it doesn't have the range of apps that other platforms have.
Google, on the other hand, would have to decide if it was worth their while to not sell their apps (maps, now, search, play movies, play music etc) through the Microsoft store. Actually they might well decide it was worth it for the advertising revenue, just as they sell some of those apps for iOS.
Selling a platform that supports Android apps is well worth it, even if it doesn't get access to Google Play; just look at Blackberry.