I would say every concern is the biggest if we truly had a SHTF situation. Now, almost everyone faces SHTF at some point, be it a vehicle break down in a remote location, job loss at a bad time, hurricane/tornado/earthquake, etc. Very few situations in the US have ever turned into a long term, wide-scale SHTF condition, though. Certainly the Great Depression was one, and the Civil War was another. Short of that most bad situations were relatively limited to just parts of the country or were short in duration.

If we had a genuine SHTF/EOTWAWKI event? Honestly I think all bets are off. The food supply would be disrupted, there might be no water, no fuel, no communications or transportation systems working, no medical system or infrastructure, etc. Some of you might have read the works of Selco, a fellow who lived at ground zero of the Bosnian collapse of '92. The cities were encircled by battling militaries that prevented the delivery of food and supplies. The hospitals all collapsed, there was widespread murder and looting in the total absence of govt and law enforcement. He now teaches courses which could be interesting but maybe not useful. His lessons applied to his situation which will likely not be yours/ours. For instance, while stores were closed the US and NATO did airdrop millions of MREs over the course of the crisis. Certainly if the US collapsed there'd be no one dropping food to us. Also, his writings reveal that a lot of his survival came down to pure luck (although he never directly admits it).

It's pretty hard to answer without defining "SHTF". If we simply skip ahead to the worst, some unspecified yet utter collapse of Western civilization, then I say make peace with whatever force you worship. I don't anyone currently alive has ever experienced that before, and I can't imagine any preparations that would suffice. You could store years worth of food but how would you defend it for years? You could attempt to "bug out" to a remote location but where in North America could you hope to go that no one else could find or reach you? You could stockpile common (and even uncommon) drugs and medications but it's still a lottery; get cancer or have a heart attack and you simply will die. Get a bad injury and your odds are poor. The US military can use a small truckoad of supplies to stabilize a few soldiers, and they have somewhere to evac the wounded to. Where would you evac to? Can you do a heart transplant in your bunker? Do you need a perishable drug that can't be stockpiled (like insulin?).

I guess I'm not sure what the point is. Obviously any preps you do are good, especially in light of the fact that most emergencies are limited in scope and limited in duration. If there's a hurricane or tornado, or an earthquake, eventually the clouds will disperse and the ground will stop shaking. Recovery might take a long time but there will be resources or at least places to go to escape the effect.

But what can you do if everything collapses? I honestly have no idea. No matter how well you prepare it will likely come down to luck.