Nokia's problem was their UI, which was not controlled by Symbian, not the underlying OS. Hundreds of Symbian-OS based phones were on the market from 2001, including in the Japanese market which had the most testing. The p800 and p910 had a UI similar to iPhone and Android in 2003, 3 years before the iPhone, on 3G networks. The second version of the Symbian OS kernel from 2003 was real time i.e. a comms stack could be run together with the apps off the one CPU. Are the Linux kernel or iOS kernel real time now even?
Android OS and iOS are hacks of desktop OSes so are optimised for performance rather than battery life or memory usage; Symbian OS was designed as a mobile OS from the beginning optimising power and memory management.
It was the timing of Google's IPO which scuppered Symbian. Google were able to spend billions getting Android in place to capture Symbian's market share and customers as iPhone buzz grew the market.