Any free, unregulated market will eliminate competitors until they create an oligopoly or even a single monopoly. It's an immediate consequence of the rules of the market.
In reality, comparative advantage and specialization dynamics do not uniformly equilibrate on monopoly markets.
The entire food production and provision supply chain has razor-thin margins. If you're hyperfocusing on that industry, it's a sign that your heuristics are pointing you at the wrong things.
https://en.wikipedia.org/wiki/Standard_Oil
Carnegie Steel: Government intervention. The federal government imposed steep tariffs on imported steel, shielding domestic producers like Carnegie from foreign competition. Without those tariffs, cheaper British/German steel would have kept Carnegie’s dominance in check.
Southern Pacific Railroad: Government intervention. The federal government gifted it, through the Pacific Railroad Acts, millions of acres and subsidized loans. This state-sponsored land monopoly let it block competitors from critical routes.