You should actually say that a specific Nokia device stays in market for 2 years max and then another version comes... But if you look at the E series they have maintained the same form factor and the OS gets updated... I just ran spotify from a E61i even though it was intended for a E63. Different phones, same OS...

With Maemo will be the same and better... Qt will actually bring both Symbian and Maemo on the same level of development. If it will succeed is a different matter, but the idea is great IMO.