Although there are no technical obstacles to alternate roots, and alternate roots need not imply duplicate addresses, inertia stops people re-tuning their Internet settings to alternatives. This control provides a rare choke point over the Internet. It can be leveraged to do good things, like ensure there is competition among domain name providers, or very bad things, such as forcing other participants in the system to facilitate or even enforce content controls. Everyone, except perhaps the monopolist created by earlier US policies, agrees this is a good idea in principle.
Corporations have shareholders and competitors.
Many non-profit organizations have members who can challenge corporate misbehavior. A third-party beneficiary agreement is a deal by which two parties agree to provide a benefit for someone else. If either party reneges, the third party can sue even though he didn't sign the contract or put up any money for the benefit. The large number of beneficiaries makes it an unusual use of this legal device, but so long as the beneficiaries are identifiable registrants, the odds are that most courts would enforce the deal. Privatized international governance is only an better than ordinary government when the new governors are at least as accountable as the old ones.