We don't know the details of the contract between MS and Nokia, so you're just speculating. It's the same thing as when everyone was thought WP will be exclusive to Nokia, yet HTC and Samsung came up with their own devices.
I never said I want Nokia. What you described can be applied to any manufacturer, not just Nokia. Why is LG making Android phones when they are no different from Samsung's? Somehow they still manages to sell 10 million smartphones in Q1'13. The same can be said about Sony: Why are they still in the business while HTC and Samsung "pretty much got everyone covered"? I guess they don't.
You are right that the competition in the Android market is fairly tough, but for some reason all the major manufacturers are still selling more smartphones than Nokia. Sure, Nokia's sales have been getting better but the pace is way too slow. They are making loss quarter by quarter and that can't continue forever.
I would argue that there's no room for three major ecosystems in the market. If we go back in time and look at the PC industry, Windows and Apple's OS have always been the major OSs. Why? Because they were the first ones to be widely adopted. In the case of smartphones, these OSs are iOS and Android. Most people just want their phones to work, they don't want to spend hours learning how to use the thing and tinker with all the nice little animations. The problem is, for most people iOS and Android are "good enough" so they don't see the need to switch. Manufacturers have also realized this as Nokia is the only one who is putting some serious effort on WP phones, though MS is certainly shooting at their own leg with all the stupid limitations - with Android there's more freedom and that allows OEMs to differentiate their products (and makes the market more appealing).
I'm just very, very skeptical about the future of WP. To me it seems like MS thinks they can just ride on the Windows name and sooner than later people will switch to it since that's what their computers run. The WP platform needs to be better than iOS or Android to really gain popularity, but that's not going to be an easy task as iOS and Android are already pretty darn good. Or maybe I'm just tired of all the Nokia kool-aid everyone drinks here in Finland...