> And life likely wound up that way because its precursors—hydrogen and carbon—are readily available, it’s energy dense both volumetrically and by mass, it’s safe and it can be converted into various forms of energy directly and easily.

Yes. Most of the available energy is in the hydrogen bonds, but if you want to 'tame' neat hydrogen by binding it to something else, from all the stuff in the periodic table carbon is pretty much the optimal choice.

> Ultimately, the big unknown is the cost of synthesising Jet A. I think it will be low enough that it blows the hydrogen hypothesis out of the water, but I don’t have more than a hunch to go off.

To the extent one of the inputs to synthetic Jet A is hydrogen, it won't be cheaper as such. So the question really is whether the added cost is low enough that the easier logistics of a room-temperature liquid fuel makes the total cost lower. I'm slightly hopeful that this will eventually be the case.

If we can get it be unleaded in the air - and perhaps a little less loud… I live on a river and also a jet path. It’s like a freeway at night and also the residue from the jet exhaust is rather annoying to clean off the boat…

Jet fuel has never contained lead. Lead is still used in aviation gasoline, used by small propeller planes (larger propeller planes typically use turboprop engines that use jet fuel). But consumption of aviation gasoline is a very small fraction of the consumption of jet fuel.

Jets have actually declined significantly in loudness over the past decades.