While shares of retail brokerages such as Robinhood have been under pressure, they have hardly collapsed. In part, that is because payment for order flow is just one of a number of ways that brokers have historically—or could in the future—made money from trading flow.
For emerging brokerages there are other non-fee revenue streams available, like cash management, or lending stock out to other market participants. But customers may have various issues with those things too, such as seeing their holdings facilitate short selling. In the end nothing can be truly free, including trading.
https://www.wsj.com/articles/why-free-trading-will-never-be-free-11631871038?mod=djemalertNEWS