After starting this thread and reading your helpful replies ,I have some positive thoughts:
What do we need exactly from Nokia (except another one or two firmware updates)? almost nothing!
-After seeing windows 3.11 / windows 98 / android running on N900, I'm almost sure that maemo 6 will be ported to N900 somehow unofficially.
when this happen, the whole problem will be solved (although this depends on the degree of compatibility after porting)
-Even if maemo 6 will not be ported to N900, currently Qt4.6 is being ported so maemo 6 apps written using Qt4.6 will be compatible with N900 and this ensures that n900 will have access to new apps after releasing maemo 6 and will not be dead as we fear.

So I think that the worst case of Nokia to ignore N900 is not a big problem.
Am I right in these thoughts or I am too optimistic?