EdB

The water/methanol (not ethanol) mix might help civilian turbo-charged planes, but not the NA planes, which still make up the brunt of the fleet. I am not sure water injection would lower the need for higher octane, which is still made with lead. I think the boosted planes you name also used 115/145 octane for detonation control. Water injection was only for short burst of power needed in "emergency" situations, like limited to 5 minutes or less. Fluid tanks only held 30 gal or less. I have heard that some of the engines were "required" (think probably didn't happen) to a mandatory tear-down following full use of fluid. I don't think that would entice many aircraft owners to keep flying. 

We have an IH 806 gas(!) waiting for update to include installation of a Keystone turbo system. Keystone offers/recommends a water injection system they sell for use during full hp needs. But again, the key here is turbo boosted. I doubt that putting a water injection system on a non turbo engine would do anything except lead to lots of rust problems. The water also has a tendency to squish out the fire.

Finally, we're back to the "small potatoes" thing. Internet sources say US fuel mfg make about 137,000,000,000 (that's in billions!) gallons of auto fuel annually. The same sources say low-life refinery folks like Shell people make 180,000,000 (million) gallons of 100LL annually, and it's going down. Doesn't include diesel. My digital calculator says 100LL is slightly over 0.1% (one-tenth of a percent) of the amount of fuel made annually. There's bigger fish to catch, though little planes make easier targets.

But, rest easy EdB. If you can hang in there until Jan 2031, you'll get to see California ban the sale if 100LL in the state.