If the FED didn't step in, every regional bank in the country would experience a bank run as people would withdraw everything and deposit in the "too big to fail" banks for safety.
Why I think the depositors should've suffered a haircut: What the FED did, was implicitly guarantee the deposits, this incentivize banks to become even riskier with deposits as they get to keep the profits if their risky bets payoff and get bailed out if they fail. This is like a real life cheat code for bankers and unfair to the rest of us regular folks who has to suffer the consequences of our actions.