I'll preface this by saying that SAF by itself isn't a silver bullet that solves all problems with carbon use in aviation. It can, however, be an important piece of a larger solution. Additionally, even in isolation without a larger plan it has a net benefit on carbon reduction which is a win in the battle against climate change.
The basic problem is that “sustainable” aviation fuels, if based on biofuels, would substantially compete with food production.
Certainly possibly, but not absolutely.
Virgin feedstocks (the stuff needed to feed in to make SAF) would support your position because the plants grown specifically for harvest to be turned into SAF would displace food crops, or possibly support destruction of other non-agricultrual land to grow net more crops. I agree with you that both of these situations would be a net negative to SAF.
However, virgin feedstocks aren't the only nor even most desired feedstocks for SAF. There are many ways to produce the fuels that fall into the definition of SAF. Things that we would otherwise consider waste streams can be SAF feedstocks such as the following:
There are other pathyways being explored too such as the waste water runoff from dairy farms and beer breweries:
"To that end, the Argonne Lab scientists look to using carbon-rich wastewater from dairy farms (and breweries, for other reasons) as feedstock for SAF production. The study author at Argonne, Taemin Kim, said that the energy savings come in two ways. “Both [dairy farms' and breweries'] wastewater streams are rich in organics, and it is carbon-intensive to treat them using traditional wastewater treatment methods. By using our technology, we are not only treating these waste streams, but [also] making low-carbon sustainable fuel for the aviation industry.”"
Unless we as a global society choose to simply eliminate air travel for people and cargo, we have to accept that a better approach to energy used for air travel is needed to meet reality. SAF is an important part of that in my mind.
