The pre-90 debate is many faceted...
One line of arguement is that bikes are not strictly defined "Classic" until they are 20 years old and typically we have tended to jump in year groups of 5 or 10. So maybe there is an arguement for pre-90 racing to start in 2010 and call them Classic MX.
Another arguement is that the sport of Vintage MX was really about the older technologies of bikes, i.e. the transition from old British fourstorke singles through to two strokes, long travel suspension and then ultimately water cooling and disc brakes. Arguably, not much has improved, technology wise, since about 1988. (and don't sprout off about modern 4 strokes, they are a retro-grade step as far as I am concerned
) Thus the arguement is that Pre-90 is an "old modern bike class" and not really vintage racing as such.
The bottom line is that if enough people want to race pre-90 motocross bikes, why not let them? The only issue that then arises is whether they should be in Specialised Vintage MX clubs or just race in normal Motorcycle clubs as a Classic Class.