And most of the feasible scenarios in which banks start collapsing so quickly and widely that depositors end up with barely anything left bar the FDIC insurance

And most of the feasible scenarios in which banks start collapsing so quickly and widely that depositors end up with barely anything left bar the FDIC insurance are scenarios in which the economic outlook is so grim your startup that's relying on rapid growth and further funding to survive isn't going to make it anyway. Admittedly, picking a startup as a bank increases that risk somewhat, but so does picking a startup to provide your CRM system or web platform or other mission critical stuff that's a lot more likely to be shut down with minimal warning, or indeed choosing to raise funding from a VC that wants you to 20x or bust...