It is absolutely in the collective market's best interest to retain confidence in the bank infrastructure of the nation. There were already people lining up at First Republic over the weekend, Signature Bank was shut down as part of this same announcement, and almost certainly more would have come in the following week.

The collapse of a ~$200B bank was going to have significant ripple effects that would almost certainly have exceeded the fees that banks will pay to fill in any shortfall resulting from this backstop, whether they're ultimately charged down to account holders or not.