Nokia found the way to kill Symbian ...
First a correction/clarification for a post above, UIQ3 was/is Symbian. UIQ was one of at least three UIs that Symbian was supporting up until the Nokia debacle (UIQ, S60 and the Japanese FOMA phones)
UIQ was the SonyEricsson UI (for those with history degrees there was (way back when) UI Quartz and UI Ruby among others, and UI Quartz was the one that SonyEricsson took on) and was based on having a touch screen.
Series60/S60 was the Nokia choice and was designed for "one hand use" (put your own punchline here!) so mostly for the 12 key phonepads, but also for the Blackberry style phones.
So (as I see it) the downhill path that happened was that Nokia bought out the rest of the existing Symbian partners, and then imposed the "Nokia development/project management" tools on the Symbian team. Up until then, the core OS developers were fairly nimble, highly motivated, and doing a great job with limited resources, and then watching companies like Nokia stick on a badly designed UI and create a Frankenstein's monster of a phone, with incompletely developed apps and UI, which made the core OS look poor and buggy, when it was much better than that. (I loved my SonyEricsson P910i and P990i ... did everything that the first couple of generations of iPhones did (except multi-touch), faster, cheaper, more reliably, and years before)
So once Nokia were in charge, the same great management processes and procedures that had made S60 really flaky and poor, got applied to the core OS. A key year was lost in converting over from Symbian tools and processes to get everything to work on Nokia. During that time a lot of key effort also went into trying to build bigger, better, linux build systems (so lots of fixing file naming and such so it would compile on a linux based gcc and use Eclipse rather than CodeWarrior and MS C++ or whatever) ... and this was at exactly the time that Apple started doing well with the iPhone, so all the Symbian developers are screaming "we had most of these features years ago (bluetooth, cut and paste, touch screens, downloadable apps, video conferencing, wifi, OTA updating etc.) what the f*** are Nokia up to?"
And then another year was spent cleaning up code to go to the Symbian Foundation ... taking out proprietary IP that could be licenced to SonyEricsson or Motorola, but that couldn't be released as opensource code. Rewriting IP stacks. Changing header files etc. And then Nokia transitioned to Agile ... so from about 2007 to 2010, the Symbian developers were working flat out, creating things the market wanted (like dual sim phones, gesture operated phones, 3D screens etc.) and then watching Nokia kill them off at the end of development and not release them to the market (or release them in some limited buggy fashion)
I still carry an E72 next to my Samsung Galaxy ... it may not have the whizzy screen ... but I only charge it about once a week, and it never crashes ... I have to change battery in my Galaxy at least once a day as it eats battery power, and the charger socket in the phone has failed already.
From near the end of the Nokia Symbian time I got a C7 ... lovely little phone ... reset four or five times a day and would just switch itself off for no reason. Would often reset if I tried to answer an incoming call ... so totally unuseable as a phone. I think I had a worse one than average, but that really soured me on Nokia devices.