100LL is not going to disappear anytime soon.
There are too many planes around the world, and in the USA where it really matters, for it to disappear.
I think that eventually a "100UL" fuel will take over. They have already been developed.
At least I hope so, because we don't have any half decent avtur engines. Mr Thielert, ably assisted by Diamond, has thoroughly p*ssed on the diesel bonfire. The Austro engine is too new to be anywhere near proven, the SMA likewise.
We need another 5 years at least to see a decent picture of these engines. We especially need to see a good number of them out of warranty, at which time real problems (if any) will be freely and honestly discussed on the internet, whereas in-warranty the owners keep their mouth shut because if you do your dirty washing in public, the aviation industry cuts you off from factory support (as I well know).
The only other option is a turboprop, which is a great solution but pricey. You start with a Jetprop (PA46 conversion) which new is $1.7M and a good used one is about $1.2M. For $3.2M and about $1.5M you can buy the rather more solidly made TBM700/850 but that is another 2x higher operating cost and being > 2T you pay IFR route charges too. Nobody has successfully made and marketed a "small" turboprop, yet.
Avgas
is a real issue for those touring around southern Europe. A lot of the bigger airports are run by idiots who have priced things to push out GA, and as a result their avgas sales have dropped way down, so they stop carrying avgas... so even if you are happy to pay the £200 landing fee because it is only a one-off visit, you can't refuel there. There is a general shortage of avgas airports anyway, especially if you want Customs as well. In some cases a local aeroclub has found out that they can buy avgas in drums for a lot less than the local oil company franchise
(there
is a lot of money in selling avgas, contrary to what everybody tells you) so they do that and then.... the airport stops selling avgas because there is no demand but.... the aeroclub doesn't sell avgas to visitors (except via a backhander). For these reasons, Diamond looked such a super option for serious European pilots ... until their engines started to pack up.