Actually it is wrong to say that Supermarkets "used" commodity price increases to justify inflating the cost of products to protect shareholders. Moreover your reasoning about these increases having limited impact on final product cost is entirely specious. Consumers have only been exposed to a proportion of the actual increases that have taken place, with most being absorbed by the manufacturing base. Don't misunderstand me however, I'm not saying this to defend the supermarkets as personally I have serious issues with some of their operating practices. What I am saying is this;
1. Food is cheap in comparison to what it could or arguably should cost
2. Commodities and raw materials comprise a much bigger proportion of product cost than most people are aware
3. Supermarkets have for the most part avoided increasing food prices to avoid a loss of sales and thus revenue
Now as for the likes of Aldi, Lidl and Iceland seeing increased custom that is primarily down to three things;
1. People are no longer stigmatised for shopping there
2. Price
3. Minimal quality difference compared to the major retailers
The first is a consequence of falling living standards brought on by the economic downturn and is entirely understandable. However, the other two reasons are where it gets interesting. Aldi etc. have a much lower cost base than the primary retailers due to the way they operate. Thus they can sell products at a lower price and still retain a profit margin. Part of this is actually the fault of the retailers due to how they package their products both on shelf and during transit. However, where they really messed up is with quality. The reduction in quality differential has been tacitly approved in order to ensure continuing supply in the face of commodity price increases. What this has done however is create a situation where the consumer can no longer appreciably tell the difference between them, or in fact that the quality is worse. So based on price they ask themselves "why should I pay more for something that is only as good".
Ultimately the food industry needs a massive overhaul in order to deal with the challenges it will face in the coming decades. However, the most significant barrier to that is not big business it is actually the consumer. Who in real terms knows next to nothing about what they are shoveling into their mouths every day...