Retirees think of Social Security as a contract, but it’s one that Congress has the right to alter, reflecting changing needs and economic conditions.In a recent column on the senior-citizen lobby, I noted that Social Security is often “middle-class welfare” that bleeds the country. This offended many readers. In an email, one snarled: “Social Security is not adding one penny to our national debt, you idiot.” Others were more dignified: “Let’s refrain from insulting individuals...