Nokia's market share was already dropping rapidly when Stephen Elop was brought on board.
Dropping in terms of relative market share, yes, but still very healthy in terms of absolute sales and profitability. That was a long-term problem for Symbian, but not necessarily for Nokia because they were already starting to phase in its replacement (Maemo/Meego).
What Elop succeeded in doing was to turn a long-term problem into a short-term crisis. Specifically, rather than allowing Symbian to continue its slow, market-driven decline he forced it into a nosedive, while at the same time switching its dedicated successor from a product that was ready to ship to one that would not be ready for some time.
It is true that Nokia's development of Maemo/Meego had been proceeding far to slowly and inefficiently, and that at least cannot be blamed on Elop, but it is hard to imagine how Nokia could have done worse by sticking with Symbian and Meego compared to the spectacular decline that resulted from adopting Windows Phone.