There is a number Of reasons for and against the pre '75 cutoff date. If we move it from pre '75 to pre '78 then we include rear disc brakes (slight) increase in suspension travel and 'lean forward' rear shocks.
The reason the RT15's are allowed is because they are easily changed to Pre'75 specs and once that is done, they look the part and most are already fitted with pre '75 eligible engines.
On the other hand I agree that an RT15 is light years behind the likes of an '84 EML.
Whenever you include cutoff dates, no matter where the goalposts are set, usually the latest allowable model within that class is the best, especially with sidecars where there is 10 years between '75 and '85.
If we change it to pre '78, then the guy that owns a 1969 model looses out.