How has hardtail MTB geometry changed in the past 10 years? So I have been sitting on my 2005 Specialized Rockhopper because it is such a good fit (23" bike I'm 6'6") however I probably haven't ridden it in 5 years as it needs a new front fork. I'm just wondering, is it worth keeping and replacing the fork or will I get left in the dust every ride or laughed out of town?
I'm not sure about the geometry, I doubt that it's changed much. But I suspect that the question you really care about is in your last sentence.

You've got a perfectly good bike, other than the busted fork, and it fits you. A newer bike will perform better, but it won't be a huge difference. If you haven't ridden off-road in 5 years, your atrophied skills will be holding you back way more than your equipment.

If you want to buy a new bike, and you have the cash, then by all means do it. But don't let them con you into believing that everything becomes obsolete when the new model comes out.