Name one country which is fully self-sustaining.

Agricultural subsidies exist primarily due to lobbying, the "we don't want to starve" argument is nothing more than an excuse used to justify the fortunes handed out to corporate-scale farmers.

Even if one grants this, it does not make the case that any particular set of subsidies is justified by that reasoning.

It is also the only alternative to a granary system to smooth out the variability of yields each year that might not average out for anything less than 10-15 year spans.

And the granary system regularly still resulted in shortages and famine. While crop subsidization has a bullet proof record of surplus.