You are overestimating XNA. Most (almost all) high quality games for Xbox and PC are written in C++. It is even more likely that they could be ported to iOS or Android more easily due the simple fact that they allow native code. XNA would be a competitor to the popular Unity game engine which at the moment has a more competitive platform to build for (webplayer, iOS, android) and also offers a superior IDE while offering the same language support as XNA. In addition to that, UDK and the ID engine are also being ported to support iOS and Android. Feel free to point me to a list of high quality in XNA produced titles.
Good game developers are able to create games regardless of the quality of the tools. Its part of the fun to create impressive games with limited resources and tools.
Other than marketing the fact that Microsoft also created a successful game console and xbox live, I don't think Microsoft has any advantage here. Xbox live could be a key differentiator but only after a significant market share has been gained. I don't think people will buy a WP7 phone for Xbox live. Currently iOS offers the best games.