In an act of pure indulgence, regulators have given the five banks nearly six months, until Oct. 1, to fix their plans. That is on top of a reprieve that the banks won in 2014, when the F.D.I.C., but not the Fed, rejected the wills of 11 banks, including four of the five banks that were rejected on Wednesday. Back then, the regulators’ disagreement led to a decision to give all of the banks more time to revise their plans.

The longer it takes to develop living wills and enforce them, the bigger the risk of uncontrolled crises. But living wills are not sufficient by themselves to ensure financial system stability. They do not fully account for the ways that the failure of one bank could cause the failure of another and, in that way, become a systemwide problem.

Ditto for today’s higher capital requirements, which are an important regulatory tool for controlling risk. But the way capital is calculated does not fully account for the risks inherent in big banks’ holdings of derivatives. One of the reasons that Bank of America’s living will was found deficient is that the bank did not have a sound plan for winding down its portfolio of derivatives in a crisis. Neither Bank of America nor the other big banks are required to hold as much capital against their derivatives’ bets as is required of big international banks.

Living wills are an important piece of the regulatory puzzle, but only one piece. Constant vigilance for systemic risks and bigger capital cushions for derivative holdings are just as crucial.