I've held-back on entering this discussion, I think as riders of certain bikes (Pre 95) appear so will classes for them. I do tend to agree that LTR bikes will chop a track up quicker than a bone-jarrer, so maybe not combine the classes at riding events but I really want to see them together at display events.
I definately feel there is a place for Pre 90 & Pre 00, anything that is not current and has an enthusiast following should & will have a place.
Here in Vic it's crazy having three bodies for older motocross bikes, suely CSC should combine with VCM and cater for up to Pre 78 or maybe Evo, and Viper should cater for Pre 80 (Evo),Pre 85, Pre 90, Post 90 2-stroke, Pre 2000.
I think Viper pretty-well cater for those classes save Pre 2000 (they do have a modern class), Once a few Pre 95 & 2000 riders appear I am sure Viper will include them.
Hell I enjoy a '98 RM250 just as much as '69 DT250MX.