The established supplier would just buy the new competitor. If they don’t sell, the established supplier would pricedump and eat the loss, since they’re already estabished, until the new competitor agrees to be bought.

How would they get the capital together (a massive undertaking) or the equipment together (again a massive undertaking) as Walmart buys up the entire supply chain to provide the pipes and power lines?

Oh you want to hire an electrician for grid work? Walmart won’t sell you or rent you the equipment. Want to hire a plumber for grid work? Walmart won’t sell you the required equipment to set it up.

Not even that but what’s to stop them from corporate espionage? Sabotaging their competitors. “Oh we trap rain water and truck it to people. Lately people have been getting really sick from our water but not Walmarts water.”

But their only customer would be the one store Walmart is shutting down. Why would you lay millions of dollars of pipes and more millions of dollars in purification infrastructure just for one money strapped store?