Market power is the power to determine where resources are allocated. While it is true that expanding the size of the cola market can help both companies, that is a different concern than the one in which, say, Pepsi is losing market share. The concentration of spending power means that resources allocated for the rest of the population are dropping in their share of the total amount of resources available. I am not arguing against centralized democracy, whatever that means As an aside, the amount of centralized democracy is what the writers of the American constitution had to grapple with. If it is the latter, I have not understood your argument.
Stock prices reflect the first, because they often result in dividends for the financier.
Profit in the first case can be increased by reducing the pay of the employees. In the second case, the survival and behavior of companies does not have to be determined by the stock market. Instead, it can simply be determined by consumers.