It am still astonished when I remember Nokia buying (the rest of) Symbian in June 2008. This seemed a bizarre waste of money. And so it proved. (The purchase of Trolltech was earlier, but surprising nonetheless.) Perhaps if they had done so years earlier, and open sourced it, then things might have been different.
My opinion on the failure of Symbian was the failure to enagage developers. That, and the most tortuous development environment and deployemnt process that I have ever witnessed. Perhaps this could have been fixed, given time, starting earlier, but corporate institutions historically don't behave in ways to allow this. Regardless, it was much to late by the spending sprees of 2008.
Nokia should shunt Symbina into the siding now and adopt Android immediately. It should get back to making great phones.
[0] It has its place in some phones, arguably, but how long would you choose to continue to develop and support it when there is no long term need.