Why are Social Security’s protections irreplaceable?

The best part about Social Security benefits is that, in contrast to savings in a 401(k) or IRA, they never run out (until surviving children become adults). Furthermore, in contrast to a pension, they are fully portable between jobs. And in contrast to both 401(k)s/IRAs and the vast majority of pensions, benefits are protected from erosion by inflation, through the COLA.