Geoff, clearly I did not spend enough space clarifying that riders can win against newer bikes...Kevin Windham had a go on a 77 Maico and went better than the others on their moderns. I rode a practice day a few years back at a local modern track-a very SX type at that-and went well on my old 77 Maico( not due to my skill, which is zero-but familiarity with the bike)....in early 90s Joe Busby here in Tx won a vet race on his 72 400SB.
On the right track, the right rider will prevail, we should all be aware of the 90% rider 10% bike saying. That said, I am confident my old 77 Maico -on a rough track would hammer through the rough line that has a 75 Bultaco, Monty VA, or 76 CR 250 for example kicking the rider off. So, I agree that almost all 75 bikes-and many 76 bikes would have riders disenchanted-they already should be. They already don't have a class.
A NW US group had proposed a "Trans-Am" class for these 75 ( and select 76) bikes. Most of the 75 bikes are at a techincal disadvantage to the top 77 bikes now.
What I am saying is that the 78 KX 125, and 250-78 YZ125, 250, and 400, 78 RM 125, and 250( if not also the 78 RM400) are so technically similar to their 77 counterparts they present no technical adavantage. While it is the rider, I -if riding say a 76-77 KTM, 76-77 RM, 76-77 YZ, 76-77 Bultaco, 77-78 Monty VB, or 76-77 Maico AW am not intimidated by a 78 KX, RM or YZ on the line...and right now these 78 YZs in particular are comparatively cheap-meaning a few more blokes even with less $ could participate and grow this class. Here in the US, it needs more participation.