Not to mention their Office 365, MSDN, etc lines that are mad profitable.
1) Apple makes a fraction of its revenue from the app store, about 8%. And they're considered one of the two, if not the most, successful app store on the planet.
2) PCs, as opposed to mobile platforms, are open systems with a radically different distribution culture. You visit a website, click a button to launch an installer, choose a directory and go. Yes, the customer would prefer an app store, but giving MS a 30% cut when you drive all the traffic and marketing and downloads on your websites anyway? Unlikely. On Mobile there's not much of a choice.
It's obvious that a windows store will generate revenues and would be a good move. Mac and Linux have em, and if well executed on the biggest PC/Laptop platform in the world, then it can be a big deal. Just look at Steam, somehow it drives so many eyeballs that publishers are willing to give a cut to Steam to do something they're already doing themselves on their own website easily: distribution.
But steam being the favorite by far, still makes between 1 and 2 billion dollars in revenue.
In short it's pretty unlikely to see MS really thrive on app store revenue. It'll be a decent product, but it's probably more for improving the customer experience (through a properly curated store) and get people to like Windows again than to make a ton of money. For MS it seems it's still all about selling licenses to vendors, office and enterprise/education. It's kind of funny how extremely non B2C it is, and how B2B focused it really is, for a company that builds products used by billions of people. I think these latest moves have kind of shown MS understands that and is trying to get closer to (dealing with) the customer: build their own hardware, make their software free.
Combined with their Azure services, they've developed several other valuable streams of revenue. Probably enough where they can give away their OS for free in order to gateway these users onto their other products and services. This is a classic business move, the same thing Apple has done for years with its products.
Granted, they're not the biggest, but they are starting to put a dent in Amazon's market:
http://www.infoworld.com/article/2608726/cloud-computing/clo...
"Our commercial cloud revenue grew 147 percent this quarter, driven by both Office 365 and Azure. Our commercial cloud annual revenue run rate more than doubled this year and now exceeds $4.4 billion and with this rapidly growing scale we continue to expand our cloud gross margins."