The other answers have covered most of the issues such a company would face, but here is another important one I don't believe has been covered:
How would such a company control its employees?
Unless the company is going to freeze hiring and force all existing employees to continue working under NDA agreements in perpetuity, the secret isn't going to stay secret for long. Presumably the company would have to be very compartmentalized, so (for example) the janitor and the secretaries don't have access to sensitive material, but there is still going to be a strong incentive for corporate espionage. What if the chief scientist who developed the procedure decides to jump ship? If key people in the company start "disappearing" (whether because the company decides to eliminate possible leaks, or because outsiders kidnapped them) this is going to affect morale and make control even more difficult.
Those who are most knowledgeable about and closest to the process in the company also are the most likely to be able to surreptitiously apply the product to themselves, or spirit away quantities of either the ingredients or final product without detection. Is the company going to treat assembly line workers (or whoever is involved with the process by which the product is made and packaged) like diamond mine workers with invasive searches and the like? There is certainly going to be some turnover in such a job, and thus a constant stream of new workers that are potential threats to the company.
Update: As @user3082 points out, the company may try to retain a tight control over its employees by incentivizing them to remain loyal. 159
This could introduce an opposite dynamic to the diamond mine analogy. Some employees would have been loyal without any incentives, some employees are going to be happy with the incentives, and some are going to be greedy and want more (and maybe more after that). The company is either going to have to engage in some sort of incentive discrimination, and hope nobody who finds out cares, or potentially pay a lot to keep everybody happy, and this could start to seriously eat into its ability to "rule the world". For instance, in the extreme case, everybody in the company could share roughly equally in the company's success. Now even if the world is being ruled by the company, it is also being ruled by committee, and whether or not it would be effective or whether and how disagreements between members could be resolved could cause power struggles and instability on its own. 140