Bringing in Qt, and PIPS (P.I.P.S. Is POSIX on Symbian OS), in a Eclipse based IDE (Carbide), helped make the development experience much better, but then came Elop and ruined everything, with management's board help.
It was pure facade to say "continue with Symbian because PIPS + Qt". There were a lot of strange quirks in PIPS because the underlaying system/kernel wasn't unixish. And there were a lot of things all unixish systems as of 2008 already did well (multi-CPU, large memory) which Symbian really struggled with. Some of the "small-system optimisation" in Symbian (the single global service message port / queuing system) made it very hard to parallelize/scale out.
Nokia somehow was so uncomfortable with the idea of an opensource "base" that they rather bet on / sold out to Microsoft. But then, this is water down the river, and we will never know whether "alternatives" to the bet they made would have had better returns.