problem was not symbian? would agree to that for the most part.
problem was 150 phones? disagree to that. they sell huge numbers of phones all over the world. different markets and different users have different requirements and i always could find something that fitted exactly what i wanted in their hardware range. better than the apple one size fits all policy.
i think their problem was the way they kept changing symbian so that addon software from one year would not work on next years phone. the platform while it didn't change much in look and feel changed enough so that every year i found i had to find new software to do what i was doing on last years phone. this pissed off developers who would often release only once or twice before abandoning the platform. in the time i had symbian devices i went through at least 4 different etext readers as the old software wouldn't work on the newer devices.
they never cared about creating a developer friendly platform. pity as it remains the only device i could write and run python software on. ase for android is getting better but is still not as good, yet.
just my 2c.