There’s a lot of talk these days about overhauling the retirement-savings system. In Washington, much of the discussion centers on ways to give an immediate boost to government revenues by limiting retirement tax breaks.

One idea that gets discussed is to repeal the current structure of pretax contributions to retirement accounts in favor of a system where contributions would come only from after-tax income—as contributions to Roth IRAs do now. It’s an idea, called Rothification, that has been championed at times in the past....