Take a look up North to see what happens when grocery conglomerates into a single empire.
Loblaws and it's umbrella of corporate enterprises not only control the majority of the grocery sector but also is vertically integrated throughout the supply chain from the production and transportation to sales. Their assets are so large that they are able to leverage them to also provide financial services. They own an astounding amount of retail spaces including drugstores (Shoppers Drug Mart), supermarkets (Loblaws, No Frills, T&T, and many others regionally), and megamarkets (Great Canadian Superstore). It's a pretty contentious issue.
When the government went to ask them to lower prices because they posted record profits and clearly overcorrected for inflation, they were basically get told to get fucked.
They were also found guilty of price fixing bread and baked products, not that the fine equated near to the amount of profits made by it.