In the meantime Symbian is far from dead: My N8 is just as usable and relevant as my N900 or any other device:
We must understand that the problem with getting Windows Phone for Nokia going was going to be one of slowing the all-but implacable momentum of the giant that is Symbian. As the Elder OS, Symbian might crush the younger and currently less-capable Windows Phone. Symbian is huge, a monster with a life and trajectory of its own - that is why Elop has done all he can to be seen to 'kill' Symbian. As if he could!
Nokia will be far more than a manufacturer of Windows Phone devices - that was the deal on offer from Google if they adopted Android. Nokia will work with Microsoft and other participant manufacturers to build and shape Windows Phone.
Your last comment is very true: Nokia will not be the same ever again. It has been toppled by fast-growing competition since 2007 and came close to being unrecoverable. But this is a dynamic and unpredictable industry; who could've predicted just four years ago the rise of the Android, the growth of Samsung, HTC, Huwawei and ZTE, the fall of Sony-Ericson, Motorola and, of course, Nokia.