> if two competing companies do their marketing in an optimal way, it will leave the customer in the same state as if no marketing effort ever occured

Marketing covers more than this. It isn't just how the pie is divided up, it is also about increasing the size of the pie.

In fact, if it increases the number of potential consumers, marketing can be rational and profitable even if it reduces the market share of the marketing company (50% of 10 consumers is less profitable than 40% of 100 consumers). There is a real rising-tide-raises-all-ships effect, especially in discretionary goods (eg, jewelry, many electronics, financial services, the list goes on).