...based on the assumption that your shelter survives intact and no outside factor at all interferes with your noble attempt at sustainable long-term farming.

Unfortunately, that is rarely (if ever) the case. The problem with natural and man-made disasters on a really large scale is that they tend to turn the entire society upside down. Your home, belongings, food stockpiles, the entire infrastructure might be gone in the blink of an eye.

Not just your home, the entire region might no longer be a safe place to live any more. Maybe only for a limited period of time (think Katrina or the recent Nepal eartquake) or maybe for years on end (Chernobyl, civil war in Syria). You might be dealing with rapidly spreading epidemics, rampaging violence, loss of your beloved ones and all your belongings.

Fortunately, humans are an adaptable species and most of the time we manage to fix things somehow as long as we stick together. In any sort of disaster outside help is always extremely important - in terms of supplies, rescue personnel and rebuilding resources. Without that sort of assistance, your chances of long term survival drop toward zero regardless of your skills and stockpiles at hand.

In case of a total long-term collapse however, your best bet is getting the heck out of there. In a warzone like Syria these days you either take sides and fight or let go of everything and escape while you still can. Anything else is likely to get you killed quickly, including any attempt to sit out the bad times by keeping to yourself on your well stocked homestead.