Contrary to your opinion, Maemo/meego was ready for market, as the N900 showed. Their big problem, and the reason that platform appeared to fail, is because they refused to actually install it on any other of their phones. Now try to think about why... was it because the platform was bad? Nope, everybody who owned an N900 raved about it. It was an awesome platform. Period. So why would they hesitate to market it? Probably because MS told them to, fearing that if it really came to market in a big way, it would destroy their chances of using Nokia to push their substandard shit.
The other consideration was the timing of things. Do you honestly believe that the MS "deal" happened at the same moment that the public announcement was made? Of course not. They were restructuring and getting ready for it YEARS before. Of course, they didn't REALLY plummet until AFTER the announcement. The one single simple announcement that instantly told the entire world that Nokia was a GUARANTEED FAILURE..... and they tanked.
Do you remember who else did the deal with MS? It was PALM. They had a platform that was getting a bit long of tooth (remind you of symbian, maybe?). They did the deal with MS, and DIED. They broke it off with MS and tried to claw their way back up, but too little too late and they have now become a footnote on wikipedia. Now same thing is progressing as expected with Nokia.
The MS platform is wholly unappealing, period.