The iOS7 design is close the what Windows Phone 8 has. This convergence in mobile OS design is actually going to help Windows Phone.
A strong Windows phone is great for mobile computing in general. However, I find it doubtful that "design convergence" as you call it will help Windows phone. I think Windows phone is a great product, but it will take Microsoft re-connecting with consumers to drive sales. Their most recent advertising has just been confusing... not inspiring desire, IMO.