A) The web has always been platform dependent - from the days when devs would program for Firefox and IE6 separately (and then the fun days of IE6, IE7 + Firefox) to the modern delights of which browser/phone/pad/laptop combination are we using today.
B) Microsoft's platforms have always been closed. Arguably the main selling feature of Windows is that developers who write for Windows produce products that either don't work at all on other OSes, or work pretty badly (with a few exceptions).
C) Microsoft is a platforms company - as is Apple, as is Twitter, as is Google. In fact, Facebook's entire valuation was pretty much entirely based off the back of "Facebook is a closed platform". If it was just a place to store your photographs and chat with your friends it would have a total value in the small millions, not the billions.
You need to have your eyes pretty firmly shut and your fingers pretty deep in your ears and singing LA LA LA pretty loudly to think that owning platforms as a way of making money is dead, or is coming to an end soon.