I've already written an article on limbo responding to that charge. That's timely because the charge is about to be revived as the Vatican's International Theological Commission reviews a draft of its pending report on limbo. The key clue to the result has already been given out by the ITC secretary-general, Luis Ladaria, SJ. On Vatican Radio, he said there is "no binding doctrine" on limbo—thus implying that it's been a mere theological opinion after all—and that "[b]ecause of recent developments, not only theological but also of the magisterium, this belief is in crisis today." I predict the eventual report will say that, while it's acceptable as opinion to believe there's a limbo, it's also acceptable to deny there's a limbo.
Beyond reviving the charge I've already addressed, that will cause many Catholics—especially women who have miscarried—to believe that all infants who die without baptism will automatically go to heaven. But as hard-hearted as it might seem to say so, such easy universalism finds no more warrant in the deposit of faith than the old rigorism. The most defensible position, it seems to me, is the one I took in my article: there is a limbo, but it's not permanent; everybody eventually gets the chance to choose definitively for or against Christ. If they've had no opportunity to do so in this life, they get to do so in the next.