What farmers “want” to do is irrelevant to the argument. What matters is what farmers actually do. Farmers may “want” to till more deeply in response to topsoil, but that doesn’t undermine the argument because it doesn’t imply that farmers in fact are tilling more deeply in response to topsoil erosion.

Not necessary, because the author doesn’t have to assume the “deeper” the till, the “greater” the susceptibility. It’s possible, for example, that deep-tilling leads to more topsoil erosion than shallow tilling, but that effect happens only after we exceed a certain depth.

Necessary, because if this weren’t true — if tilling by another method besides deep tillage IS a viable option — then the author has not proven that current deep-till farmers should switch to no-till. Maybe they should switch to some other till method besides deep-till? The author must assume that these other options aren’t viable.

The “most expensive farming methods” have no connection to the reasoning of this argument.

Not necessary, because we don’t know the relationship between greater or less aeration and amount of topsoil loss to erosion. If, on average, topsoil that is no-tilled is NOT more aerated than topsoil that is tilled deeply, that doesn’t undermine the argument, because we’d still have evidence that the deep-tillers are more likely to experience topsoil loss.