You brought it from this perspective, while other members questioned the survival of TDA, since in their view - it's too late, because of Robinhood. Good question.
I know a few people (including my wife) that will not use anything else except Robinhood, simply because everything else seems too complex. All they want to see is a basic chart and couple buttons. Such extreme simplicity is what made iPhone the leader (first phone with one or no buttons?), it is what makes Craigslist so popular, and it is the requirement for any modern apps. Who cares about 20 products or websites or apps doing the same thing when people always come to the one that's simplest/easiest to understand, and that they already got used to. People don't even care about the price of products like iPhone or Tesla, it's all about usability, brand, familiarity and what their friends use. Professional traders may not use Robinhood, but how many professional traders are there, within 6 million Robinhood users?
BTW, people have been laughing at Robinhood and asking "can they even survive?" since Robinhood launched. While Robinhood has been continually turning people who never knew what "stocks" are into small investors and amateur traders. Asking "can they even survive?" today is no different than asking the same question two years ago, a year ago, or a year from now. Maybe they will or maybe they won't, but that's no different than speculating about AAPL, FB, TSLA, BYND, etc. Only psychics are professionally qualified to answer such questions
RH is basically a gambling app. That's a turnoff to not just "pro traders", but non-pros with serious accounts. As far as I'm aware they also don't (yet) offer a product aimed at the long-term investor, to compete with robo-advisors - outfits like Wealthfront and similar offerings by the old-line brokers. I don't know enough about the cost structure of these businesses to say how many $5,000 accounts are needed to make up for a $5mm account - but my suspicion is that dollar balances and being able to charge a management/advisory fee, even if slim, are vastly more important than just opening millions of accounts.