No the kindle app is not enough but the guys from MS have done a great job getting the developers of the top apps from android and ios to make WP apps. It's rumored they've even bank rolled porting some apps to WP. WP has the fastest growing app store so even if it's not Nokia developers someone is writing apps for the platform and that is all that counts at the end of the day.
Your 2nd point about royalties doesn't make much sense either because we all know that Nokia isn't paying much due to the contributions it is making by bringing Navteq maps to MS. The exact deal isn't know but it is speculated that Nokia isn't paying anything. Even if it were it's better to be pay $5 per phone and selling millions than paying nothing per phone and gradually selling none