What is the difference between a 1989 CR and a 2000 CR bike ? The answer is , not much . So you just cleared that matter up yourself
Quite a lot, actually:
Frame material and design, size of the brakes, suspension design, long stroke motors, much more sophisticated power valves, much better carbs and ingitions, ergonomics, wheel size, etc etc.
Even a nuffie rider like myself can feel and appreciate the difference.
A modern MX rider would laugh at the idea of riding a ten year old MXer.
I know that the next argument will be "but a good rider on a pre-90 bike will be able to beat a lot of riders on current machinery!".
This is true, and I don't argue with it.
However, if you think that's a solid argument against pre-90 as an old bike class, then you should look at the fastest pre-70 lap times from the Nationals and see where those bikes/riders would have finished in the pre-85 class... I think you'll find that the only logical conclusion is that we've been wasting our time by seperating bikes by eras since 1988.
OR, the seperation of eras isn't entirely about competitiveness, and has a lot more to do with showcasing eras of bikes.