I see what you're saying, and as said its would be hard to stop and in some cases would be silly. However the likes of Sainsbuys and Asda tried to link up last year and that was stopped, they said it would be for the benefit of the customer, wider choice and cheaper prices, what a load of tosh, it was so they could fix prices and reduce choice.
But then you have the like of B&Q, Wicks and Homebase ( cant see Homebase lasting much longer) putting small DIY shops out of business, all three of them can name their own price, and there's sweet FA we can do about it. That said more importantly if they fold up, Were do you go?? so not great that they rule the whole market.