You make some very valid points. Society hasn't progressed to the point we are at today by acting on a every man for themselves mentality.
One thing the Prepper types underestimate is what it is to be human... Every crises man kind has faced in the past we faced by banding together and overcoming it. Look at this nation post 911.. Europe during WWII with victory gardens, neighbors feeding neighbors... Post Katrina 1000s of Americans opened their homes and housed victims all across the country.
Last year we happened to be in VT when they had their major flooding and people where without power, houses washed away, schools flooded out... We saw people pull together and start cleaning up as soon as the water receded. Business that didn't have power to operate emptied their freezers and held community BBQs without charge. 4x4 clubs hauled water and gas into places that were cut off from access to town etc.
You are right. No one is an island. It would be difficult to survive on your own. We are all very dependent on each other, even on a global level. I wonder if we could even have a true world war knowing that most countries are inter-tied.