It does exist, it has been approved by the FAA, and you can go buy the STC right now if you want[0]. The incentive is that 100LL has been banned by at least one county[1] with states[2] trying to follow (although the AOPA successfully neutered that one[3]).
They don't need to develop an alternative to 100LL. Oil companies make it in such small quantities it's a total pain in the arse to cope with.
If you adapt a car engine (like a VW Beetle or Subaru engine, both flat-fours and common lightweight GA power plants) then it just runs on ordinary 95 octane unleaded, the exact same stuff you put in your car.
