Most of the fleet (~70%) would be OK switching to 94-octane unleaded. The biggest problem is that Congress/states have made it extremely difficult to source this type of fuel, because the overwhelming majority of fuel is produced for automotive usage. Ethanol and other mandated additive blends are unacceptable for aviation usage, and we're talking about 0.1% of the fuel market. Congress/states have put up barriers that impede a shift and they need to do something to help that overcome those barriers, not just ban 100LL.
Also, due to the nature of general aviation this is a very distributed problem. Most general aviation occurs at relatively small airfields/airstrips and there's many factors that make it difficult for GA pilots to use large airports (limited access to those airspace types, high traffic levels from commercial aviation, high ramp/fuel fees designed to discourage refueling, etc). So you've got to get 92UL available at a large number of small sites. Most small airports will carry at most two types of fuel, 100LL and Jet A1, and many will only carry 100LL. Most airports are running on super thin margins and adding 92UL would mean not carrying 100LL.
That leaves the 30% of aircraft that can't use 92UL pretty high-and-dry. Some can be modified, some will need engine replacements, a lot will be out of luck. Unfortunately they are likely concentrated at the affordable end of the fleet, and that translates into more cost increases in a sector that's already dying due to a cost spiral.
General aviation is a feeder for commercial aviation, and that's going to become an even tougher and more expensive path to follow. Going commercial requires a large number of flight hours which are typically accrued by getting an instructor license and training casual pilots - so cost increases that reduce casual pilot time percolate very directly into a reduced supply of commercial-eligible pilots. The only other route is ex-military pilots, or paying >$100k through a university (which is not commercially viable due to the ~$20k average salary of an entry-level regional pilot).
General aviation is also a neat cultural touchstone that I am sad to see shrivelling into nothing :(
Re being a commercial feeder, that's like lots of things, if individuals start refusing to bear the cost of obtaining hours, airlines will be required to make changes. They might increase pay for pilots, lobby for changes to the licensing process or provide alternative ways to get those flying hours (I'm not sure how you subsidize hours without perverse incentives, but I guess you could protect investment using loans that had forgiveness for service).
You've pointed out some purposes that GA fulfills, but you haven't addressed what harm those lead emissions are actually doing. It could be minimal. Or, it could be pretty severe when you look at the aggregate impact. At a structural, societal level, poisoning people to avoid higher direct costs for air transport is likely not a good trade-off, so it is necessary to look at both sides of it.
A few nits I'd pick. Even though 70% of the fleet could burn 94UL, that represents well under half of the gallons burned. Turbocharged engines (often in twin engine airplanes) fly far more hours than the average 7.5:1 compression piston single. Yes, flight school aircraft on the low end fly a lot, but most piston singles average under 75 hours per year and often at 8-9 gph. Turbo singles, IO550s (Cirrus and Bonanza), and turbo or IO550 twins are typically burning 1.5x that per engine and flying 2-3x the hours.
I think most of the engines that can't be easily modified to run on 94UL are concentrated on the expensive end of the fleet. Most 182s/172s/Cherokees/G-cats, etc can either already run today under the Petersen STC or could be easily modified. It's the Bonanza, Cirrus, Golden Eagle, P-Navajo, etc that need 100LL (or GAMI or similar) and can't run on 94UL. Those tend to be way more expensive than the lower end of the fleet.
I do realize that in this case the harm is likely extremely difficult to even estimate.