This only works when there are economies of scale. If there are significant dis-economies of scale smaller firms can out-compete the large firms.
The large firm can't just buy up all the small firms either because new firms are started all the time, and if every person who starts a small firm is rewarded with a buyout, the industry will attract more and more new firms until the large firm can no longer afford to buy out new firms.
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Examples of industries that not dominated by three or four large companies: mining, farming, house cleaning, and being a land lord.