This is actually very common in rural parts of the us fwiw. There places where the only grocery store is Walmart, and the only other place to shop is dollar general.
In this case there is likely a limit to what a private business can grow into before it wades into the territory of public utility. A tenuous but related situation can be seen in trademarks where Kleenex has actually run entire ad campaigns to make people call generic tissue "tissues".
There's a larger philosophical argument here. Theoretically speaking there should be no limits to private corporations. However, when your corporation becomes a stable of modern life and the conduit of major utilities of society all of the sudden things start to turn into shades of grey.