Windows was a horrible choice for flagship OS, agreed. However It is important to understand that Symbian is not the ultimate OS to have. Symbian is better than Windows Phone because it probably at the moment has better apps and flexibility. However I strongly disagree with the point made on the article that Symbian is better than Android. Really? Why do people deny the pure fact that Android is the only true OPEN os that lets people do whatever they want and its evolving and available for many devices. Don't get me wrong I am not an Android fan (Give me Maemo 5 Community edition any day) but it is probably the best os out there right now.
Also Nokia necessarily did not have to move to Android they could have stuck to Linux based OS technology and introduced them to Symbian lovers...trust me after thousand groans and moans they would have realized what a great OS a linux makes.
So my point is Nokia needed to evolve in the OS department that's for sure whether with a more advanced version of Symbian, stick to Maemo Meego or even Android. If they stuck to Symbian they would still have that niche market. With Android they would have a more mainstream market. With windows phone they have close to zero market.
But bear in mind an organization always needs to evolve.