Eventually the winner enterprises get so big that it is prohibitively expensive to enter the market in competition with them, and smaller firms simply become auxiliary forces for the large firms, to be absorbed when the charade of separateness is no longer useful. Once all firms are vanquished/absorbed/made into auxiliaries, you have a vertically and horizontally integrated monopoly enterprise.

Lemme tell you about a lil' something called "Innovation"

Under communism, there is no innovation because everyone is forced to do the same thing or they get shot. Under capitalism, innovation is rewarded by 1 person becoming a billionaire and everyone else also gets richer because capitalism is when people have money. Do you want people to have no money? No innovation? I know what it's like to live in a communist country (I was born in Ukraine in 1999) and let me tell you, you would change your mind very quickly if you experienced the reality of it.

If there was a monopoly under capitalism, the people would simply vote with their wallets to promote healthy competition because capitalism is synonymous with democracy. Arguably the only thing more democratic than capitalism is the blockchain. I have been huffing lead paint for the past 3 hours straight.