Is efficiency the best health metric for an economy, or is it just an easy one to use? Many of the most useful things become natural monopolies because they are so good.
Consumers are also producers and most consumers don't want to spend their lives being faceless cogs in the giant corporate machine but they don't have a choice anymore. What is the point of satisfying the most trivial consumer needs when their most fundamental needs to have a meaningful job are overlooked completely?
If there were fewer corporations, people would have better jobs, fewer people would live in big cities and they would be happier overall. The system would be more efficient at delivering value to people.