As long as the owners of Nokia’s Symbian devices can go to Ovi Store and download new applications for their devices and therefore make their devices feel fresh again, Symbian can stay alive.
As soon as Nokia pulls the plug off Ovi Store (shall it ever happen), Symbian is dead. After that, I’d imagine most Symbian users will abandon their devices and try something else.
Just my two cents.
For now, things are OK. Even for Qt developers.
If you are working on a Qt application for Symbian, there is no reason to give up. Finish your project. Put it in Ovi Store. See what happens.
Today there is as large a market for Symbian apps as there was before Nokia’s announcement that they will move to Windows Phone platform. In the future there will be even more Symbian^3 devices out there. The market continues to grow.
Realistically of course, after a couple of years, the penetration of Symbian devices will not grow anymore as Nokia stops producing new devices. The penetration starts to decline, as people move to other devices. At that point every Symbian developer should have learned to develop for some other platform. Maybe Qt has been ported to Android by that time, or maybe Windows Phone proves to be an exciting platform to develop for.