You are mistaking what is insured here. Deposits are insured, the banks themselves are not.

Here's the list of banks that have failed in the US and for which their deposits entered receivership managed by the FDIC; looks like a handful every year: https://www.fdic.gov/Bank/individual/failed/banklist.html

I'm not. I was trying to point to the OP that banks aren't as fragile as he think they are. When the very nature of your business, which is handling people's money, is federally insured, you have mitigated a lot of risk.

Yes banks can fail and they can go bankrupt, and if that happens to Nubank then this acquisition goes to hell which makes the Clojure ecosystem weaker. I give that to OP.

But saying that banks are "very very" fragile institutions I think is fundamentally wrong.

The fact that deposits are insured has nothing to do with bank solvency. People just get some of the money back when the bank blows up. Arguably this makes banks to be even more reckless and to hide more dangerous risks under the deposit insurance claims.

And the function of the regulator is to stop and DISSOLVE the bank BEFORE it goes potentially too deep into the pocket of deposit insurance fund in case of running astray. So this is a social instrument but again does not solve the institutional risk problem. Maybe, just maybe makes some banks more paranoid, but they still blow up at very high pace.