What also killed Symbian was it's app signing requirements, adding additional burden to small companies and startups. Also, when Nokia saw that others are a better deal for developers, they still didn't have a backwards compatible plan on transitioning to their newer platform - Maemo.
Microsoft's Windows Phone on the other hand was had two entirely separate versions: Windows Phone 7 - just an UI layer on top of Windows Mobile, hardcoded for 480x800px screens, and Windows Phone 8 - a new kernel, a new graphics system, but Microsoft made sure older apps can be ported to the new system. What killed Windows Phone was Google and their refusal to support any app on their platform: Drive, Docs, but especially Youtube. Microsoft didn't have Office 365 at the time.