the question I'm replying to was about why the need for fuels at all, not specifically Hydrogen. I'm not big on hydrogen. I like the ideas around ammonia based fuels. Biofuels have other environmental costs that concern some people. Synthetic fuels could serve a purpose, but yes energy cost has to be really low for that to make sense. But they might be the only viable option in some cases.

Obviously costs change the demand side of the equation - it may be that we wind up with very expensive airfare, and have a lot more somewhat slower modes (say high-speed rail) that can more easily use the cheaper energy forms.