Whats good for consumers isn't whats good for the broader market, we wouldn't see mega billionares with net worths close to that of previous captains of industry if we forced them to allow others to compete (where the only viable way to do so from my perspective is to not let them compete at all). In every other industry you see far less vertical integration due to the history of how companies have been regulated against (i.e. Car and airplane manufacturers are using very few of their own parts).

Society doesn't really care whether there are 10 megabillionaires or 100 minor billionaires.