Don't most of the more popular aircraft have STCs that allow them to run on automotive gas?
2. It only applies to low compression engines, which is a lot of engines, but which also rules out most airplanes made since the 70s. There's a few exceptions, but not significant in terms of manufactured numbers to matter. (the STC you're likely talking about is the one that let engines run on 80 octane)
If the MoGas STC mattered then pilots would have adopted it because rec fuel (ethanol free gasoline) is significantly cheaper than AvGas
#2 and the low availability of mogas at airports is the reason for a lack of adoption fleet wide.
That's the premise/promise of G100UL: it can serve all the spark-ignition piston aircraft.
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.