I'm not interested about Symbian phones really these days but Symbian is THE OS in mid and low end. Nokia packs lots of hw and features for 150 euros Symbian phones that's not really possible with Android yet because it's a resource hog.

If anything it's very possible that Symbian sells in 2 years time in same numbers as S40(it's not Symbian!) as in Nokia's plans Symbian takes S40 place. When people here say Symbian should just die i don't think people get that if that happens Nokia will essentially be next Motorola.
Old part of Symbian is AVKON that was made in late 90's for hw button based phones, but that changes as Qt takes it's place.
It will be the price and features that will sell S'4 also in future, but the great thing will be that it will be connected to the whole ecosystem that was not there with S40. The feel of the UI will change alot that i can tell already when the throw AVKON out even if they would want to keep some Symbian elements.