I think the point is that removing functionality doesn't mean you're going to move faster. Mail.app removed functionality but has received no major new features for years, and is still overall kind of a crappy phone-it-in email app. Almost anything is better than Mail.app today.
I know it sounds plausible, especially considering the ugly hacks that are revealed on The Old New Thing. But I don't think there's ever been any proof, or even good examples of it. Management, friction between departments and time wasted on visual overhauls could easily be bigger factors than a few thousand (or millions) lines of ugly code maintained for backwards compatibility.
Empirically, the iCloud reboot is still meh. iMovie (cleanly rewritten one version ago) is averaging at 3 stars on the Mac App Store, with especially devastating reviews in the German MAS. On the other hand, iOS 7 is moving very fast while being one of the products where Apple cares a lot about backwards compatibility.
"Apple removing features is what allows it to push forward so much faster than Microsoft"
In the same way that Mail.app has made CRAZY progress since RSS has been kicked out to everyone's applause... sigh