The market leader (that is, Apple/iOS; the numbers are irrelevant, they feel they're the leader today) is usually hostile to number 2 but friendly to number 3. While number 2 is usually hostile to number 3 - look how Google tries to bully WP.
(For example, MS used to bash Linux but kind of threw a bone to FreeBSD)
And this translates from companies to companies' customers.
I think you're being extraordinarily optimistic. The latest numbers I could find quickly[1] show RIM with more than double the share of Microsoft, and with both showing a reduction in share vs. six months prior. You might be able to claim that people are abandoning RIM faster than they're abandoning Windows Phone, but even that is arguably because RIM is falling from a higher initial position.
The other problem is that .NET makes no sense for mobile apps, because neither of the two largest mobile platforms have good (if any?) support for it. Everyone who is not some kind of Microsoft evangelist is going to support Android/iOS first, which means they're not using .NET and are not going to worry about a Windows port unless Windows Phone gets enough share to be worth worrying about.
[1] http://www.engadget.com/2012/11/30/comscore-android-us-marke...
There's also, ironically, giant gaping holes in the app/feature selection for business users. No Citrix, no VPN, no app that lets you get to OWA mail if ActiveSync is disabled on your Exchange account, etc.
Time and time again, making a good product, sticking to the mission and making it the best it can be, has resulted in success for that product. So if Windows Phone is a solid product, and Microsoft sticks to it, they will have a very good chance of becoming a force to be reckoned with.
In each of those cases the new product that gained ground offered something significant that the existing product didn't. Android offered comparable quality for less money. MacOS offered a significantly better user experience than desktop Windows and also leveraged a higher level of consistency in user interface between mobile and desktop for the new generation of iOS users. And iOS did the same to RIM from the other side.
None of those are simple cases of comparable products gaining ground against intrenched competitors, they're cases of superior products gaining ground. And that's Microsoft's problem. They've produced something comparable, not something superior. Where's the killer app? The UI is neither abominable nor spectacular. It doesn't run desktop Windows programs. It isn't any cheaper than Android. What should make me want to buy one of these over Android or iOS that makes up for the lack of apps and the training cost of learning a new UI?
I have both system and I just got a Mac to be able to code for iOS. I don't find your statement true at all. Actually I have a better experience with Ubuntu (as buggie as it feels) than MacOSX. For instance, in my case, I never got used to the Dock / Exposé way to switch between windows. Maybe I am just too used to the task bar but whatever the reason is, I suffer a lot anytime that I need to work on a Mac.
On the /third/ hand, I find myself more and more tempted to download VS2012 every day and do some development of my own.