Scott3:
You have absolutely hit the crux of the issue with your first sentence. It is not possible to get rid of the TEL, and maintain the same properties at a price point that is competitive with 100LL.
[snip]
Personally I think a fairly high octane, ethanol free MOGAS is the likely outcome. (Either 98Ron, or 100Ron like NPD sells in the sough island).
Some modern planes with rotax brand or similar engines will already be approved for this.
Some engines will require approval
Some will require modification (hardened valves, compression reducing pistons etc), then approval, and will take a hit on peak power.
Some engines will not be viable to modify (or will result in a power loss such that the aircraft no longer preforms as desired), these will need to be re-powered or retired. It is possible that high performance General aviation will become the domain of turbine powered aircraft rather than piston.

When 100LL becomes unavailable - which IMHO will be a short time after the USA Environmental Protection Agency 'screws its courage to the sticking place' and bans al leaded petrol - it will mean the end of the road runway for a large percentage of the GA fleet.
Every replacement non-leaded-fuel engine will require an STC (Supplementary Type Certificate) to be certified for every different sub-type of airframe to which it might be applied, and that's a process that costs hundreds of thousands of dollars, and might apply to a fleet of dozens to a few hundreds of individual aircraft. The US FAA has been running a programme to produce an unleaded 100LL replacement fuel for over a decade, and it has produced a few candidates none of which has so far been successful.

Since the GA fleet is almost exclusively elderly, either in years of use or a relatively recent plane mechanically identical to its 1960s predecessor, a large proportion will just have to stop flying. What percentage of those would be replaced by modern-design aircraft is the big question