Everyone has their pet era. When we can respect others' passion, we can grow.
absolutely, I couldn't agree more. At the risk of repeating myself, my only worry is that by introducing any new divisions we will have too many undersubscribed divisions. Eventually divisions will have to drop out or combine with each other.....for example, combining pre 85 with pre 90 to make room. Just because these five yearly cutoffs have fitted the mould in the past doesn't mean we have to stick with such a rigid formula in the future. Perhaps technologically based divisions will be a way of achieving division parity in the future. Whatever happens, eventually something's going to have to give or another split be made to cater for the next generation of bike technology.
Pre-90 is as important to some as Pre-75 is to Firko.
But I've changed. While I didn't seriously start racing until 1970 which nurtured my passion for the pre 75 era, over the years I've become more and more interested in the even earlier divisions of pre 70, pre 65 and over the last couple of years, pre 60. I was 8 years old in 1959 but I have grown to love and respect those old mild steel and cast iron heavyweights from that era to the point of assembling enough parts to eventually start building a 1959ish era TriBSA. I think that if we took a little sideways interest in bikes from outside of our generation many of us would find that not only are those bikes interesting in their own way, they're fun to build and race. The current upsurge in interest in pre 70 and pre 65 is an example of racers crossing over the invisible barrier into classes that they'd previously had no serious interest. The current "DT1 Craze" has woken up a surprising number of punters into joining the pre 70 class. In reverse, a staunch pre 75 mate of mine is currently planning a pre 90 class ATK and myself and my mate Jonesy are co-building a very, very trick pre 78/Evo 125 Yamaha. I emphatically believe that if we step outside of our technological comfort zone and take an interest in bikes we'd previously only taken a sideways glance towards, our VMX experience is broadened and the sport will become a more close knit and happy place.
Now there's some common ground.

My point is that you don't get people involved by demanding they step outside of their comfort zone.
I've lost track of the number of times 'newbies' have turned up at race meetings with a bike from "their era" and openly expressed minimal interest in the older bikes - but within a few meetings, they've seen the other era bikes in action and said "that looks like fun"... and then they hunt down a bike from an era that they'd normally never have developed an interest in.
One of my mates semi-reluctantly got dragged along to the first Pre-90 race meet in NSW. It was mostly the promise of racing my KX500 that got him there. Then I managed to talk him into coming to Canowindra on the promise of the social aspect and the unique format.
He now owns a Pre-75 bike, a Pre-78 bike, two Evo bikes, two Pre-85 bikes and two Pre-90 bikes... There is absolutely no way on earth that I'd have been able to talk him into buying an Evo bike straight up, and even less chance of getting him to race a Pre-78 or Pre-75 bike.
Edit: When time and/or money is tight, most people will contract back into their comfort zone.
Pre-90 is as important to some as Pre-75 is to Firko.
Pre-95 is that important to me. It is MY era. Pre-00 is inevitable - it holds much less interest to me, but I will not denigrate it, or otherwise devalue its importance to others.
Everyone has their pet era. When we can respect others' passion, we can grow.
Nath its not that i dont respect others passions , if their into bikes thats got to be a good thing 
I have seen you race eras from pre 70 to pre 90 and you seem to enjoy them all .
When i say that the technological advances were slow in the later eras im not digging them out im just stating fact . I still maintain that adding later classes too soon is foolish when the already established late classes pre 85 and pre 90 have small numbers and cant get enough to run a titles .
Joan has cleared up the 1960 to 1974 advances i think , Frank Mellings MX the big leap is a good read covering this topic .
A 1964 CZ250 is an air-cooled, two-stroke, single cylinder 250 with drum brakes, twin shocks and roughly 7/4" of suspension travel.
Just like a 1974 Ossa Phantom or Montesa Cappra.
A 1989 CR250 is a water-cooled, power-valved, two stroke, single cylinder 250 with disc brakes, linkage monoshock, and roughly 12" of suspension travel F&R.
Just like a 2013 KTM250SX.
There is a lot more to both technology and performance than a list of specifications or superficial appearance, regardless of the era.