There are many "prevailing wisdoms" on the future of avgas, depending on which axe one wants to grind, and the one I go for is that America will lead the world with "100UL".
The fuel already exists but for some reason it is happening slowly. I think the main reason is that avgas shows no sign of disappearing in the USA, and shows no sign of disappearing in Europe any quicker than is happening anyway due to attrition of facilities caused by suppression of GA activity via airport price increases and in some cases by aeroclubs arranging their own supply which is not available to visitors but which results in the closure of the based facility.
Another reason for slow progress is, I strongly suspect, that the developers of the fuel have filed patents and are doing what any correctly cynical businessman would do which is wait till they have the punters over a barrel before concluding negotiations on licensing it. In turn, nobody involved in the production process is going to enter such negotiations until the market is absolutely ripe.
It is by no means assured that the banal tree-huggers' challenge in the USA (which is not based on the slightest evidence of population risk, because the lead involved is so tiny) is ultimately going to shut down U.S. GA. The USA has a long history of "deadly legislative crises" which run for years and then get swept away at the last minute. I know the saying goes that the USA always does the right thing after exploring all the alternatives first, but anybody betting on the USA screwing itself in a major way is going to lose a lot of money.
Worrying about avgas is a bit like worrying about the IR. Basically, if you need a certain capability
now, you need to get off your backside
now and get on with it. What this means is that if you are happy to do £100 burger runs, and you are happy with what the actual hardware will be, buy yourself something that can run on ~96UL or better. If you want something with a FL200+ ceiling, long range, all the proper IFR stuff, then get yourself something with an IO540 (or similar) up front
All kinds of thing can cause unexpected depreciation. Lycoming's SB569A depreciated every decent IFR tourer by some £20k. I very much doubt that any retrofit electronic ignition (or even a full FADEC) system, providing a ~96UL capability for today's 100LL-only fleet (which is
big in the USA) is going to cost that much.
Actually GA has very few options. If you want to avoid avgas, the cheapest proven reliable way is a
Jetprop which is about $1M used, $1.5M new. If you are poor but you feel
lucky then you can buy a DA42, which has a spare engine