It will be better if you are willing to pay the overhead airports and FBO's have to pay to maintain two dispensing systems. You think Avgas is expensive now, wait until you have that kind of capital investment. I don't think you have really thought this out. New tanks, new dispensing equipment, double the number of trucks. Oh yeah it would be in the billions. Even worse, you would be amortizing that equipment investment over half the gallons, because while the total demand has not changed, you have double the equipment.
Lycoming can wail all they want, but they told the FAA when they got a TCDS what they expect in exacting terms of fuel and the FAA took them at their word. When a fuel came along that was not on the TCDS but met all of Lycoming's expectations it informed the FAA during certification of their engine, the FAA said yes. Unless Lycoming wants to go back and amend their TCDS to specifically say "this fuel, with this formulation will not be used" then the STC stands. Further in doing so, Lycoming will have to explain why so that the FAA can exclude other STC applicants and amend their process which is a very, very tall order because it will basically upend the certification of all their engines, and they know it.