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.