This is somewhat wrong on both counts - MeeGo will be released for the N900 in the form of the Developer Edition, and while the name Maemo 6 might be not be in circulation, the Maemo 5 successor project is very much alive - as announced by Nokia, will be released as a MeeGo related product later this year. Nokia might not release that one as an upgrade, but I'm confident there will be community efforts to backport as much as possible from it to N900 in some form, within the limitation of the hardware and various drivers, of course.
Whether by Nokia or not, the current N900 adaptation team and those of us that are enthusiastic about MeeGo on the N900 will keep the DE platform updated. You have to understand that MeeGo development and also Nokias' N900 Developer Edition are all being developed in the open, so even if the first DE release (based on MeeGo 1.2) is all Nokia actively contribute their resources to, the project will still be alive. In fact, Nokias participation in creating the "Developer Edition" is merely assiging Nokia devs work and making sure MeeGo contributers are concentrating on the right areas, bugs and features, to reach the DE specifications. And it is clearly visible that Jukka and the team have successfully made really great progress in this regard. I can only hope we continue to have the great project management we've had recently with the N900 DE project. Something that has been a great help and source of inspiration for us all.