Here is a list of aviation industry businesses
who will be definitivly destroyed if 100LL is outlawed:
Everts Air Cargo (has some turbine planes also, but they stay on the few paved runways)
For many of their customers, a different option does NOT exist.
Others operate mixed fleets, they will loose their piston powered component:
Basically every airplane owner with Radials, Lycoming or Continental engines will be forced to to technical modifications, that are not yet known, explored, legal, tested, etc etc
In any way, expense will be huge, IF THE TECHNICAL POSSIBILITY EXISTS.
Private (public service possibility) and part91 (within a company ops):
2xGrumman Tracker and Trader
How would you like it, if someday some agency shows up and after 60years of use says you cant use your vallued property anymore, because some radial special interest group lobbied successfully to outlaw your fuel?? or a component thats only 2 parts per million of it?