Central bank buying govt debt or QE is basically just maturity transformation. The Fed / ECB / BoE buys 10 year debt and issues cash which is also a form of govt. debt. IMO, when interest rates are as low as they are, it is foolish to issue short term debt. A lot of people like Martin Wolf and John Cochrane have been calling on govts to take advantage of the low interest rates and issue perpetual (or 50 or 100 year) debt and insulate themselves from sharp increase in inflation or interest rates.