"The advantages of the OS are increased security and massive parallelism, both due to the fact that .NET code is inherently memory safe."
So, what? Everything in userspace on an Android phone is Java and runs in the JVM. That's not really a feature users care about, but even if it were, the leading OS already has it.
You then go on to say:
"Remember that although us geeks are picky about the OS our phone runs, a the large majority of phone buyers really don't give a crap"
Which I agree with. Which is why there's no reason for someone to switch to a Windows phone, even if, on some technical level a .Net phone were lovelier than the competition.
Just because the next magical version of Windows is based on a smarter core, doesn't mean any normal user will have a reason to switch.
"On the contrary, they're just getting started, and this deal with Nokia is going to be very valuable for both companies."
You still haven't answered why Nokia would sign on with another sinking ship rather than simply choosing to make great devices for the leading (free) OS.
Of course, because the new CEO is coming from Microsoft, he may very well have loyalties that simply don't make sense for his new company...and if that's the case, he'll merrily drive the company into further insignificance while drawing a nice fat paycheck, and making his friends at Microsoft happy.