How has hardtail MTB geometry changed in the past 10 years?
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.