If they had concentrated on Series 60 they could easily have fixed Symbian over time. It was highly capable. For example it ran a full Webkit (yes really) browser before Apple shipped their own. That was genuinely useful. It even had code signing for apps in v3 (the version that shipped with the E61)

The problem was still Symbian under S60, if you like. Yes, it had code signing (which seemed like an unnecessary restriction at the time it was introduced) and a decent browser, and email that synced in the background (unlike in iPhone 1!) and background tasks in general, etc.

But developing for Symbian was convoluted, very painful and slow. And it slowed down Nokia itself not to mention the 3rd party/external app developers. There was no reasonable way to fix Symbian as these issues stemmed from the very foundations. One of them being memory management, the other probably cooperative multitasking and callbacks. But the memory management thing was all over the code (think string handling, so everywhere) and it made using existing software hard too. Linux would have been the way to go, one way or another. Sure, they would have to have rebuilt most things for that platform but e.g. webkit would have been a no-brainer and they could have used a lot of existing open source software.