In engineering, "design life" is the period over which a system's built-in assumptions about the world are expected to hold. The Army Corps completed the Hurricane and Storm Damage Risk Reduction System in 2018 with a 50-year design life, meaning its 350 miles of levees and floodwalls were sized against projected subsidence rates, sea level rise, storm intensity, and coastal wetland extent through roughly 2073.
Those projections assumed a cushion of up to three feet of freeboard. By 2019, the Corps acknowledged that cushion was shrinking fast enough to threaten the system's 100-year protection standard "as early as 2023" without periodic levee lifts. The agency budgets $3.5 to $4.2 billion for those lifts through 2073. Beyond that year, no engineering plan exists. The Corps says "additional evaluations will be needed."
So 2073 is not when the levees fail. It is the last year anyone has committed to doing the math. The distinction is real, and less reassuring than it sounds.