The trouble with this critique is that it takes as a premise that you have a government which is enforcing at least contracts and property rights, but not antitrust laws. Without the former you can't have a monopoly because people would just take the monopolist's stuff (though of course then you have different problems). With the latter, contracts for anti-competitive mergers are illegal and consolidated markets get broken up.
The general problem, which is not limited to capitalism, is that organizations try to consolidate power. So you need something to inhibit that. What this is supposed to look like is a limited government that can enforce antitrust laws against other organizations, but is constrained from itself becoming an organization that consolidates power. This requires strong checks and balances, and they clearly need to be more robust than the ones currently in place. Not least because several of the ones originally in place under the US constitution have been removed through amendments (e.g. 17th) or creative interpretation (e.g. Wickard).
