I have no special knowledge about this, but from watching MS since their early days I don't believe their goals are quite as crass and simple as you describe. M

I have no special knowledge about this, but from watching MS since their early days I don't believe their goals are quite as crass and simple as you describe. My guess is their internal narrative is more along the lines of: "A large share of market is shifting away from PCs to smartphones and tablets. Big factors in the success of these devices have been the simplified UI and the centralized app review processs to enforce a baseline level of quality and help prevent malware. Apple and Google have major businesses built on their respective app stores. With a billion or so Windows users there are good reasons to think that our app store could make the others look small in comparison. 30% is similar to what the other app stores are taking and it will be a lot easier to reduce this number later than to raise it."