It might also end the dominance of some of the mainstay 1960s era training aeroplanes, but I can't honestly see that as a bad thing.
Great to have you on our side
It would take out all mission capability / utility value out of GA.
It would shrink GA to farm strips, mostly. I know a lot of UK GA would be happy, on the basis of "I am allright Jack".
A proper solution needs to be found.
My engine (IO540-C4) can probably run on 96 octane just fine; it's one of the marginal cases. But most of the more capable ones can't and the airframes would be scrapped.