Now that the pace of the U.S. Federal Reserve's "tapering" of its asset-purchase program has been debated to death, attention will increasingly turn to prospects for interest-rate increases.

But another question looms: How will central banks achieve a final "exit" from unconventional monetary policy and return balance sheets swollen by unconventional monetary policy to "normal" levels?

To many, a larger issue needs to be addressed. The Fed's tapering merely slows the growth of its balance sheet. The authorities would still have to sell $3 trillion of bonds to return to the pre-crisis status quo. The rarely admitted truth, however, is that there is no need for central banks' balance sheets to shrink.