The N900 has been superceded by newer hardware because that is the way it goes on mobile development but the N900 will go on a lot longer than anything brought out in the past simply because it is the first mobile computer and has turned out to be a great test bed for software developers and will continue to do so for some time yet.

It is a design started from the first quarter of 2009 and since it's release a lot has been learned by Nokia so i suspect strongly it has turned out to be the "prototype" mobile computer from Nokia in the hands of the general public.

All i can say is watch out for future devices from Nokia knowing what they have learned from the N900 as it will only get better and better.