Adding a compatibility layer for a large competitor means:
A) users will get the cool OS updates as soon as possible, but after the guys running the 'real' thing get them.
Guess what choice users will make?
B) developers who develop for your platform have to support the market leader, too; developers who develop for the market leader get your users for free.
Guess what choice developers will make?
This tactic killed native OS/2 apps and, subsequently, OS/2. The Blackberry plot didn't end that well for Blackberry, either.
This is a good point. Few developers would care about Windows Phone if they can just build for Android and do a lazy port to WP (if they port at all). But what if Microsoft instead wants to make Windows apps work on Android? Microsoft is championing their "universal apps" now, where you build one Windows app and it runs on Windows 8/10, Windows Phone and soon Xbox One. Adding Android to that list would be big.
A) users will get the cool OS updates as soon as possible, but after the guys running the 'real' thing get them.
Guess what choice users will make?
B) developers who develop for your platform have to support the market leader, too; developers who develop for the market leader get your users for free.
Guess what choice developers will make?
This tactic killed native OS/2 apps and, subsequently, OS/2. The Blackberry plot didn't end that well for Blackberry, either.