Windows is a closed source OS but it isn't a 'closed system', far from it in fact. You can buy software from any source for any purpose and install it without needing Microsoft's approval. You can write your own software, in any programming language you like, that interacts with any/all of your PCs capabilities and install it on your own hardware without restriction. Your Windows PC will act as a USB host for any other device irrespective of the OS on the other device. Same with networking, same with bluetooth, etc...

Windows Phone would be a much more interesting proposition if it offered the same possibilities as Windows on a PC but it doesn't, instead Microsoft decided to mimic Apple's control freakery. That combined with a fugly UI isn't particularly enticing.

Another point to take on board is when you buy a PC it always has Windows preinstalled. Personally I wipe it and replace it with something better but a majority of users just live with what's installed on a device when they buy it. Windows dominates PCs by default not because users choose it. Microsoft don't have that luxury in smartphones and tablets.

Last quarter Samsung overtook Apple for profits on mobile phones too.

Here's a little teaser for you Lumiaman:
What do the Burj Khalifa, Taipei 101 and Petronas Towers have in common with a Galaxy S4?