Nokias problem right now is not poor sales or a Symbian that can't compete with Google. Even S60 v5 is competing with Android, and it is doing well in the market segment it is supposed to be competing in.
...MeeGo is also draining recourses, not nearly as much as Symbian, but much more than the potential benefits would dictate.
MS has one huge headache. WM7 is sold on devices that are too expensive on HW that is second rate in terms of build quality.
It (WP7) really has no chance in NA against Google and Apple, and in the rest of the world it has to compete with Nokia as well. In fact, the only place it sell, at least some, is in the NA. Everywhere else people are simply thinking why? Why, when I can get Android or Nokia at a better price, or I could go all Apple.
On top of this, MeeGo is lurking, an OS that potentially could blow MS out of the water all together (if all goes well).
...they have one thing in common: online services. They both want that very badly, without it they will both be doomed.
MS can take over the responsibility of maintaining and developing Symbian, the core OS, and do a really good job at it. This will solve Nokias problem overnight, and they can put their best people back into core Nokia stuff.
...Symbian will eventually be left out (in the cold) to the open source community, and Nokia will be all WM core. This may happen very fast, depending on how flexible the new WM core is and how it adopts running on low spec HW.