I really believe that this is just a mind set. Go and watch some footage of some mid-nineties bikes being ridden by decent riders through some whoops, off big jumps or around a knarly motocross track. They are quick.
I don't understand your point.
That good riders can ride bikes fast?
Seen footage of Robert, deCoster etc hammering across rough-as-guts tracks with 4" of travel? They are quick.
Ditto the good guys of any era, including 1977, 1981 and 1984.
Apart from anything else, nobody who wants pre-90 is asking for SX tracks - a good grasstrack suits old bodies just fine, thanks very much.
While there have been a few 'turning point' improvements in MX bikes over the years (most significantly long travel suspension), the reality is that MX bikes are improved in small steps, every year or two.
There's been 16 years of small steps since the last pre-95 bike was built, and another 12 years of small steps since the last good Evo bike was built.
As I've said before, go and ask Jay Marmont how much you'd need to pay him to ride an 89 or a 94 YZ250 to defend his National title. Tell him that you'll build it to the absolute limit of the rules and you'll make it as competitive as possible, and see how much he drops his asking price by... Tell him that they work great on rough tracks and on big jumps, and see how loudly he laughs at you.