To an extent, you are correct. However, as you may well be aware, there are aspects of the device which the community cannot control. Take PR1.2, for example - had the community been able to create it themselves, or at least be involved in its development, it probably wouldn't have taken so long to deliver so little. Unfortunatey, it appears that the N900 is not as open as we like to think (albeit more open than any other device of its type) and we still depend heavily upon nokia for many major improvements.

Please prove me wrong, but I'm under the impression that much of the core experience is closed-source and therefore maemo would not be able to survive without nokia's continued support.

Support which, sadly, seems rather half-hearted at the moment.