The original plan for balance sheet shrinkage, articulated by Bernanke in the States, and Mervyn King in the UK, was organised around the idea of spending as small an amount of time as possible using QE as the marginal tool of adjustment for monetary policy. This in turn flowing from the idea that the effects of QE are ill understood, so best not to rely on it if you don't have to. [I think this argument has flaws, which I'll write about in another post].