Guys, pre-90 and pre-95 or even double overhead cam, fuel injected 4 stroke classes (work out which year that is) are NOT what VMX is about.
Says who? Everyone has their own opinion on this, and "because I say so" doesn't suddenly change my mind. There's dozens of people who will argue even less liberal definitions than your's...
If nothing else, you're completely ignoring the indisputable fact that people will always want to race old bikes that they identify with/find interesting, and do so amongst their peers. As Graeme is trying to point out, this suggestion allows for that, without worrying about the definition of
VMX or similar.
The reality of the situation is that the era from around 1965 through to 1985 is derived from the great technological change that occured in the design of the motorcycles during that era and THAT is what makes it so easy to define that era as VMX.
If that's what it was about, then the cut-off should be the 1983 (when the KXes first got front discs & some KTMs got USD forks) or 1986 (when the KXes got a rear disc). There's no justification for 1984 being The End of significant bike development. Therefore your assertion that VMX (or old dirt bikes or whatever) is specifically defined by those years is clearly incorrect.
Since 1985, despite what anyone may claim, bike design has NOT progressed in the same kinds of leaps an bounds that it did in 1965-1985.
The only aspect of bike design that quickly rendered previous designs obsolete was LTR - and even then, it took five years to go from 4" of rear travel to 12" - each one of those years saw bikes with more travel, but it was still a incremental process, just like everything else before and since.
The fact that a bike like a KX500 remained competitive and virtually unchanged from 1988 through to 2003 is a blatent example of this fact. Indeed if open class 2 strokes were still the premier class in MX, it is unlikely that they would STILL have changed much today.
You've been drinking, right? Even by the early 1990s, the under-developed 500s were way behind their 250 siblings
despite the extra grunt. Why do people pay so much money for Service Hondas, if the newer bits make no difference?
Another critical area of difference between the VMX era and post 1985 is that there was a huge change in the manufacturing supremacy during the 1965-1985 period. We went from British supremacy through European diversity (anyone who has witnessed Mark Holloway's 1974 125cc MX bike collecion can attest to the diversity of manufacturers in 1974) to Japanese dominance. After 1985, not a lot has changed...
The death of Maico, the arrival of Aprillia, TM, Sherco & GasGas, KTM as a big player, Husqvarna moving to Italy, Husqvarna becoming a mainstream player, BMW's serious dirt bikes, the short-lived Suzuki-Kawasaki alliance, Cannondale, ATK, Husaberg, the rise and fall of VOR/Vertamati, the influx of Chinese bikes, the arrival on non-rubbish Chinese bikes... Yeah, nothing happening at all.