Having used both cameras, I think the image quality between the two is close enough that the larger files and somewhat improved dynamic range and resolution of the D7000 alone are, speaking for myself, maybe not compelling enough reasons for upgrading.
If the more limited feature set of the D90 is limiting your photographic abilities, then the D7000 makes more sense. If the more robust weather sealing, partially metal-clad body, better autofocus, 39 vs 11 focus points and higher burst speed are important, then the D7000 makes more sense. I guess you need to ask yourself what you need to be able to do that the D90 lacks versus the D7000.
As for the longevity of the shutter, I think it's fair to expect a shutter life at least 100,000 pictures. Some shutters will fail sooner, but many will go well beyond 100,000 shots.
Yours is a common dilemma. The question of "should I upgrade or not" has been asked many times in these forums. Another thing to consider is that lenses typically outlast bodies many times over and you can get better images with good glass on an older body than to put poor glass on a newer body. So might it be a better idea to rather buy a better lens than spend the same money on a newer body?
A few months ago in the landscape forum I was fortunate enough for one of my images to be chosen as the winner of one of the the monthly competitions - about fall colours. In the days of the D80, D90, D7000, D300, D3, D4, D700, etc., all of them newer and better bodies, the winning image was made on my first DSLR, a D70S.