I agree that would be the perfect solution but avtur burning piston engines aren't going to happen anytime soon.
Thielert have been playing with them for some years but the technology cannot be considered long term proven - quite the opposite at present.
I think it's fair to say that if you compare failure rates that cause an aborted departure, or a forced landing, on or off an airport (i.e. not just counting catastrophic mech failures), the current diesels are at least 10x less reliable than the Lyco/Conti engines. It makes me laugh when Diamond are talking about Merc engines being reliable; Mercs have a pretty lousy reputation for QA nowadays.
Maybe 10 years from now?
But look at the retrofit cost. It is totally nuts and in the average case more than the value of the whole aircraft. It's OK for new planes but who is making those? Cirrus, Diamond, Cessna, plus a load of marginal players.
In Southern Europe, Italy, Greece, Bulgaria and others, most customs airports cater for jets and jets only and have either no Avgas or at astronomical prices. Outside Europe, lack of Avgas makes trips South almost impossible.
I am not sure that is the same issue.
The reason avgas is poorly supplied in those parts (and I have been down to Greece/Turkey so I know) is nothing to do with wholesale avgas availability, which is not problematic at all. Anybody who wants to buy avgas, and has the facilities to which the company will deliver, can just buy it.
And
AFAIK anybody at all can buy it in drums, and quite a few private pilots in the further parts of Europe are doing just that.
The problem is simply that so many airports are run by stupid management which had made a decision to not support piston GA. "Piston GA is dirty" are the actual words of one UK airport manager, and "down south" having a private jet is the ultimate symbol of wealth, influence, bribe-ability, etc. Every airport manager with the MBA from some 3rd grade college absolutely falls over and kisses the runway for a bizjet to come in. And having an airline going there is like a birthday all over.
In some cases avgas is not available because of stupid regs prohibiting its sale to foreigners (Italy) or because the local aeroclub found they can buy it for less in drums than from the local BP or whatever avgas outlet, which then shuts (a couple of places in Greece, at least). In all cases one can buy it with a "private arrangement"
but that is kind of hard for a foreign visitor to do.
Africa is well stocked with avgas, once you get out of Egypt and across Sudan - I am informed. Not sure that's ever been different.