Nokia were already screwed..
Nokia were already screwed when Elop joined. Symbian couldn't compete with modern OSes such as Android and iOS, Nokia's escape strategy of moving to Maemo on high-end devices had fatally stalled with the ill-advised merger with Moblin to create Maemo. You can blame Olli-Pekka Kallasvuo for the mess that Nokia found itself in, not Elop.
Elop found himself at the head of a company with no roadmap, but still quite a lot of sales. His infamous "burning platform" memo was pretty accurate, but he was fatally undermined as CEO by whoever leaked that communication.
Getting out of the mess was always going to involve some risk. In the end he took a high-risk approach of dumping everything and going for Windows, hoping that Nokia would avoid becoming a "me too" Android player. In the end, that strategy did not work.
It was always a high-risk, high-reward strategy to tie Nokia up with Microsoft. If they'd have gone down the Android path, I am sure that Nokia would still be an independent manufacturer today.. but not a very profitable one. The low-risk, low-reward strategy.
Of course, since Nokia became Microsoft, more mistakes have been made. The last high-end device launch was over a year ago and the current product range is moribund. It's a shame because Windows is rather good, and Cortana is easily better than Google's offering.