There is also the argument that Nokia prefers to encourage contributions around the frameworks currently in development rather than encourage contributions (by opening the source code) around the frameworks that the Nokia software strategy is not interested in pushing anymore.
The developer community can contribute to community projects under development with a good chance to become a top notch media player, a top notch calendar, etc. The frameworks and APIs are all there - I would argue that the candidates as well.
You have the KDE mobile developers, the Qt Quick app developers, the Python+Qt developers, the traditional Qt developers, the new MeeGo... all of them with interesting projects in the hands and looking for contributions. Is there any reason in the N900 hardware and the Maemo 5 software to stop that coolness? Honestly, I don't see it.