Put 6 billion people in those same past societies and make them dependent on fast-food or at least populated with folks incapable of growing their own food and see what happens...

Also look at the fate of many of those past advanced societies... they collapsed at some point and disappeared or were rebuilt by others on top of the wreckage.  Even most of our "Ancient Cities" that still exist today changed hands via war or other mechanisms that decimated their populations (ebb and flow of civilization).

My assumptions start with our world as it is now - fully functional only with abundant oil and fully dependent on that oil.  If we really did have decades to prepare maybe less of the global village would fail.

The "apocalypse" I envision does not mean the end of civilization completely, everywhere, forever.  Patches will survive, and afterwards those survivors might be able to build upon the wreckage again.  

Much depends on the mechanisms of collapse, how many survivors there are, how many and how much accessible resources there are, and probably a number of other variables I haven't thought of yet or have forgot already.

As REM said - "It's the end of the world as we know it".  Not necessarily the end of the world period.

Because we require oil for the yields of modern agriculture - and that allows us to grow enough to feed "most" of our world, not all.  

"Talk to any farmer and they'll laugh at you when you suggest theres not enough food to feed that same 7.5-9 billion people."

I know few if any farmers who actually think about feeding the world's population - with or without oil.  They spend their time thinking about running their farms.

Oil goes much, much deeper than "transportation."  

Take some time to look into the oil needed for production (as a feed-stock and/or in the running of the infrastructure necessary for production) of farm machinary, pesticides, fertilizers, herbicides, medicine (veternary), plastics, tractor tires, metals for anything (wire fencing, water pumps and pipes, etc) etc, etc...  

AND then of course there is the competition for oil between the farmers and the City Saps for all of their oil needs (remember, most farmer's I know worry about actually keeping their business running and that is hard enough as it in today's economy).  

Oil is found in virtually every crevice of our civilization's infrastructure - not just transportation.  And when supplies get tight and shortages develop and wars break out...

The farmers might get dibs when it comes to rationing but their yields will go down and there will be even less to go around and that less will be at MUCH higher prices than today.