In my opinion, a clear and impending removal of AVGAS 100LL at, say, 18 months notice, would be a very good thing for GA.
Microlighting went through something similar when 4-star was withdrawn; everybody knuckled down, the various organisations worked together, and by the time we no longer had 4-star, everybody had the necessary permissions to run on EN228 unleaded. Which they still do.
Whilst I'm not much of a fuel expert, I'm quite certain that all of the problems of running light aircraft on unleaded fuel can be solved and if we were forced to, we'd solve them. 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.
In the meantime, if we all start paying MOGAS prices, if you take fuel as around 2/3 of the cost of flying a light aeroplane, and MOGAS as 2/3 of the cost of AVGAS, you'll shave around 15-20% off the cost of GA flying.
That would, despite a lot of fuss in the short term, be a really good thing.
G