I use an old school brokerage and haven't paid trading fees in many years, and most of the ancillary fees have also evaporated over time. The race to the bottom was going on long before Robinhood.
But the OP's point is correct: even with trading fees, the cost is below the noise floor. Brokerages can make quite a bit of money off of how they structure trade execution, so the top-line trade cost isn't everything.
I was reading an article recently about Charles Schwab from the point of view of an investor in SCHW and it argued that Robinhood is not that great of a threat, because Schwab already makes most of their money from things other than commissions. Therefore if they have to cut commissions to zero to compete, so be it. Brokers can make money from loaning stock to short sellers, and from interest on customer cash balances. Not to mention selling order flow - getting paid by markets to give them customer orders to execute. Schwab is trying a tactic where they encourage you to use an automated "robo-advisor" that tells you how to invest, and the advice includes a significant portion of cash, which allows them to collect the interest.
Yes, commission has decreased, there has been a fee war going on lately and as a result fees have been going down substantially across the board for the major players. Fidelity doesn't even charge for wire transfers anymore.
Do I trust this brokerage to exist in 10 years? 20? Are they going to be convicted of fraud? If I have a problem, is there a phone number that I can call to speak to a knowledgeable agent that can solve my problem?
A sampling of things that differentiate brokerages for me, price per trade is so far down my list since I buy rarely but hold for a long time.
With normal brokers, your stocks are protected by SIPC insurance in that case. It plays a similar role to FDIC insurance on your bank account. This should generally apply to Robinhood too. They state that it does.
The reason I would have some doubts is they were talking recently about providing checking and savings features like a bank, only tied to a brokerage account, and they said cash therein would be SIPC insured and the SIPC had to contradict them publicly and say "nuh-uh".
This doesn't indicate your stocks aren't safe per se, but it suggests that Robinhood may not always verify that something is 100% ok with regulators before doing it, or think about all the implications, and some day we might find out "oops" they didn't do something crucial that traditional brokers do, because they were being disruptive, moving fast and breaking things.
Another point is that while Robinhood Financial appears to be a more or less normal broker that is a member of FINRA and SIPC, Robinhood Crypto is not. This isn't exactly surprising, but again, it could make a person slightly nervous that there could be drastically different consequences someday to trusting different entities that are all called "Robinhood". Maybe people will get used to trusting both of them and someday there will be a new "Robinhood XYZ" and it will not be regulated the way people expect. Or what if when Robinhood Crypto is hacked and loses their customers assets, it turns out there is some linkage that causes problems for Robinhood Financial, even though it wasn't supposed to work that way.
Even market price trades still require a counterparty’s inverse order to execute the trade against. Thus the exchange that an order is routed to affects execution price because it depends on the available orders on the exchange, on both sides of the transaction: when and what price your order executed depends on both (e.g. if you are buying) orders placed by other buyers on the same exchange (higher bids will execute first) and the volume of sellers (if your buy order completely fills a sell order at a given price, the price will slip against you because your order will partially fill at the next higher priced sell order).
To avoid this effect one can use a limit order instead of a market order (you are setting the price instead of ‘taking’ the market price), but then it may take longer for your order to execute, and it may not execute at all if the price moves away from your limit price.
