How so? Sustainable farming has nothing to do with a man vs machine doing the labor. Moral as in we need to make sure people can keep doing this job or what kind of argument are you even making? I'd rather see improved social safety nets and job training then some appeal to history that people need to work the fields because we've always done that.
Modern industrial farming practices, designed to optimize agricultural output per unit of labor, are an ecological and environmental travesty. They require the widespread use of pesticides and toxic fertilizers that are decimating the biosphere. Monocropping unsustainably degrades soil quality. And the large contiguous areas used for industrial agricultural operations are extremely disruptive to wildlife.
The most obvious solution to the above is an agricultural system with many more, much smaller farms, where human beings do the work currently done by machines and chemicals.