As to the "app gap" once project Astoria gets up and running, you WILL see that close greatly. Think about it, I think win10 is on over 100 million devices right now? Think about the profits for the ease of tossing your app into "the project", easily developing your app for w10 and putting it in the store. Its a no brainer. Then your app is available for computers, tablets, phones all in one simple step.
Lets say you have an app for 99 cents, 1 million of those users download that app, that's a million dollars in the business that was not there before. More if you develop a free app and advertising on it. before there were not enough users of the windows store to make it worthwhile to develop a completely different app for the windows systems. Now, its MUCH easier, and you are going to an extremely larger user base. Windows/Microsoft nailed it this time!
I will say the MS store will grow tremendously in 2016. You will see most of the apps you use come to windows because there is just such a large user base in all form factors.