If it was that simple then I find it strange that these brokers don't offer free starter accounts to new users to compete against Robinhood. They must know something we don't about the real value of the company and how much competition they're adding.
Or it just may be an example of how changes don't happen instantly. If they lose enough customers, they may cut commissions, even to zero. As long as it isn't a big problem, they won't.
It seems to me that a lot of brokers are providing an increasing number of commission-free securities, and you also see promotional deals where a new account of sufficient size gets several hundred free trades.