That probably is not practical. Do you really know who owns what in the US? Do you know, for example, whether Walmart owns any of its suppliers and if so how the profit on a product is divided? Do you know how you would discover true ownership, influential shareholdings, etc? I doubt very much if you or anyone does. Teams of lawyers are employed to hide inter company relationships. The world of business, you see, is not as simple as we like to think it is.
Insurance company A is 35% owned by ins company B which consists of banking and financial interests of several otherwise disconnected institutions. Some parts of such interests might be in private hands in in the form of trusts or companies where the beneficiary is not listed as a shareholder.
You ain't gonna bust nothing unless you can get sufficient numbers of similarly interested people and similarly interested lawyers and that just will not happen.
And, don't kid yourself on this, a monopoly is a real threat. Its easier to change your government than break up a business that doesn't wish to be broken up.