Mobile Phones is S40 (not Symbian). This is the ancient Nokia platform, which is the only part of Nokia that is currently competitive in its price bracket.
The intention is to kill off Symbian as quickly and painlessly as possible. Good riddance. It was not a realistic smartphone contender to begin with. Finally Nokia stops throwing good money after bad. Only it may be too late.
One of Nokia's systemic problems is its uneven software culture. Nokia's diffrentiator was hardware. When competitors surpassed Nokia in software it was fighting a losing battle. From this point of view the move makes much sense. Remove the software that don't perform well: Symbian and Ovi service "platform", developer tools. Keep and streamline the parts that have a chance of success: S40, MeeGo.
The transition will be painful and success is far from certain, but the obvious alternatives would be even worse.