Turbine SFC is poor however, currently, with little improvement on the horizon.
Especially small ones.
They just about make sense at FL200+ due to the TAS gain, which practically implies pressurisation, e.g. a Jetprop or similar, and even then you are looking at 2x the fuel burn of a piston engine for the same airframe (PA46).
They are also very very expensive, with little prospect of this changing.
You are right about avgas. For all the gloom in the press, it continues to be available all over the world, and where it has disappeared its demise seems to have been driven by the usual European idiotic airport management rather than a lack of availability. The AIR BP booklet shows avgas available in some suprising places, with some African countries (I don't mean RSA) having loads of outlets. It is this wide availability which makes me optimistic about its long term future - albeit at a constantly rising price in Europe.