If the market cap of Google is a million dollars and a sandwich is $5 and then tomorrow the market cap of Google is a trillion dollars because they wiped out all their competitors and took over the market, the price of a sandwich is still $5 because it's quite unrelated and not affected by the number of search engines or mobile operating systems. Larry Page isn't going to personally eat so many sandwiches that it affects the global price of sandwiches no matter how much money he has. Moreover, necessities generally have elastic supply -- even if demand increased, we could just make more of them rather than raising the price -- unless you cause artificial scarcity (as we do with housing), in which case that's your problem independent of billionaires.
What the consolidated market does affect is that market, e.g. the price and quality of phones and phone apps. But that has nothing to do with what proportion of the company's shares is owned by what number of people. It's just as much of a problem if it's a publicly-traded company whose largest shareholder owns less than 1% of it. And the problem goes away if the market is competitive even if there exists someone who has billions of dollars as a result of owning a fractional percentage of a million different companies -- although that usually isn't what happens anyway because the primary driver of the existence of billionaires is "market consolidates enough to cause one company's market cap to exceed a hundred billion dollars", not "someone invests a thousand dollars each into a thousand separate companies and every one of them beat the market by a huge factor without any of them becoming a megacorp".