Meh. People used the same argument against Android less than a year ago - too late, RIM and Apple already own the market. The key to success for Nokia is managing to get their Symbian users to move to MeeGo. If Nokia manages to provide a really smooth migration strategy from Symbian (Same applications, trivial to migrate your whole profile, etc.), and if they manage to push out multiple phones with different feature sets, form factors and price points all running MeeGo, and if the first MeeGo phones are significantly more polished than the n900 UI-wise, then Nokia will succeed.
Those are a lot of ifs, but Nokia has a lot of talented people working for them these days. They still have a shot.