What you describe is the transformation of a free market into an oligopoly by conglomeration. That certainly is a thing, but not in and of itself a consequence of a market being free. The real consequence is not that free markets are bad, but that some markets need government interference to become free.
Rule one of criticizing capitalism should be intellectual accuracy, if only to ward off reactionary criticism.
I suppose you're right that it's not a consequence of a market being free, because I see it happening whether the market is "free" or not, unless there are very strict controls placed on wealth accumulation that prevent any one person or organization from gaining disproportionate influence. I think they'd need to be so strict as to be hardly recognizable as capitalism anymore, because the way I see it, inequality weathers away barriers to its widening much the same as water freezing/thawing in cracks of rock.
But this isn't a criticism of capitalism itself, so much as the slippery wordplay that U.S. free market proponents employ to avoid anything ever being seen as a consequence of their ideology.
People/businesses using their money to create/influence organizations that suppress competition and create oligopoly is not accepted as criticism of free market ideology, because their doing so makes the market not free.
Yet placing any controls on the use of money to prevent anyone from doing this also makes the market not free, and capitalism naturally provides the greatest rewards to those who find and leverage advantages, whether they're free market in nature or not.
So by these standards, which by my understanding are genuinely the standards of definition proposed by libertarians in the U.S., asking for a free market is like asking for a fire that doesn't consume the fuel it burns.