I have no experience with worn 9 speed chains, but I would expect them to wear almost as well as 8 speed. The length of the bushing that wears and the length of the pin that wears is not that much shorter than the corresponding lengths of the pin and bushing on an 8 speed chain. The reason I went with 8 speed in my builds is that there is a bit more cable pull per shift, thus the rear shifter stays in adjustment better. All my bikes that are newer than 20 years old use 8 speed. (Exception, one bike is internally geared hub.)

My utility bike is 7 speed and is stored outside, but I rarely ride that more than 3 miles per errand so that chain does not get much use. I suspect that chain life is more hampered by lack of maintenance, I think I only oiled it once last summer. Have not ridden that bike for several months now, as you can probably guess.