People still use Bank of America and Wells Fargo despite the incredibly customer-hostile things they've done in the past. I will not be surprised if Robinhood weathers this pretty well after a short term hit.
The portion of people who invest -- outside of forced investing through 401k, etc. -- is pretty small in comparison.
Those that are using an investment application to speculate on stocks on their phone are a smaller yet demographic. And I'll bet that, dollars to donuts, they're more informed about everything, and more willing to shop.
I lazily left my 401k at my previous job "running" for 18 months before finally rolling it into my personal IRA. I'm sure I'm not alone in this.
These weren't one-off incidents (Robinhood has not, yet, established a trend), these were systemic issues with these banks. And people still go to them. They're getting new accounts daily, not subsisting off of legacy accounts.