The basic problem is that “sustainable” aviation fuels, if based on biofuels, would substantially compete with food production. This limits their scale pretty significantly, so they can’t easily scale up to the levels that the airline industry wants, which means that the cost will remain quite high.
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Yes, they’re different products, but their chemical similarity means they have the same constraints on sustainable production. There is a single limit on how much of the two can be produced in total without causing significant environmental damage.