Why would they become more expensive?

Unfortunately commodities have a lot of cheap fossil energy embedded in them (e.g. on average, 10 calories of Fossil Fuels for every calorie of food, maybe as many as 50 to 1 in the case of intensive beef!)- up until very recently that energy must have become ever more affordable since we have been able to use steadily more and more and then we have used it to produce ever cheaper commodities.

Take away adequate amounts of cheap Fossil Fuel (according to the EIA data the 'low hanging fruit' has already been picked!) then it isn't profitable to produce commodities at such low prices - if wages don't rise faster than the prices they become less affordable! In the case of food, if you can't afford it and there is no adequate charity you die of starvation, a serious consequence! History tells us that the past is not necessarily a good model for the future!

xeroid,
Only a tiny part of the cost of food is the cost of energy used to produce it, energy calories are very cheap and food calories are fairly cheap. Most of the cost of food is the processing, packaging and marketing costs. A bushel or wheat or maize makes a lot of bread or cornflakes.

I don't sit in a field eating corn, I eat bread and cornflakes in a city, so it's the embedded energy in the final food on my plate that is important to my way of life - unaffordable energy means unaffordable food, no matter how low cost one particular input is.