They are going to have to change something either raise the retirement age or lower the payments
Or just pay directly out of the general fund, which they can do with a simple vote in Congress and which they already do for Medicare/caid.
The fact that social security is not tied to the market is super irrelevant
It is the primary argument both for and against the program. Investors kick and scream about the benefits of compound interest, right up until a big market nosedive and bankruptcy spree. Meanwhile, it's the benchmark for guaranteed basic income that progressives love to reference.
Decoupling income from economic growth isn't irrelevant. It's the program's entire raison d'etre.