It's probably too late for Nokia to go for Android for their top-end offer: They have a working platform for smartphones and Android will only create "internal" competition.
At the same time Nokia doesn't want to be dependant on others that Nokia perceive as competitors.
Microsoft was not perceived as a competitor but more as a partner who could provide software and services that Nokia failed to develop.
We shouldn't forget that Nokia failed horribly with their Ovi store too.
Nokia is a good example of hubris at work.
Lets remember a bit of history.
Nokia were the leader in features phone as well as smart phones (Symbian S60) and they were rightly proud of their dominant position (50% of the global shipments).
They never believed in touch screen, doing everything possible to kill them within Symbian.
When the iPhone came out they dismissed it as a failure... when it was clear they were wrong they went in panic mode.
Some says Nokia could have joined forces with UIQ to build on a platform already made for touch screen.
But Nokia did not want to collaborate with other manufacturers, that's was clear.
Nokia decided to slap some touch screen support on top of S60: the first phones were abhorrent, with the updated OS shipped too early with very little integration between the touch support and the UI.
At the time Nokia has several strategic projects going on at the same time to counterbalance iPhone and their best engineers and designers (to don't mention budget) were split between them...
several projects were axed and reborn in different ways.
When finally S60 had a decent touch support and Meego was getting mature, Elop arrived!