Exactly. You can barely sustain a fuel farm on the fuel that services 100% of the piston GA fleet. It's incredibly difficult to make the economics work to add a second fuel farm that serves only 30% of the gasoline sold, cannibalizing sales from your other fuel farm.
That's the premise/promise of G100UL: it can serve all the spark-ignition piston aircraft.
No, it wouldn't be that much higher for mogas (typically a 91-ish octane unleaded, E0 (ethanol-free) gasoline).
If there were an unleaded 100-ish octane fuel legally available as a substitute, that would have demand if 100LL were banned. Over 70% of the avgas burned is burned in airplanes that are not eligible for the STC* to allow them to burn mogas (typically as a result of having lower worst-case detonation margins as a result of being turbo-charged, super-charged, high-compression, or some combination).
* - Supplemental Type Certificate - an airplane modification, in this case a mostly [entirely for most airframes] paperwork modification, to their original type certificate.
That's the premise/promise of G100UL: it can serve all the spark-ignition piston aircraft.