Can $180 billion of monthly quantitative easing by the ECB, BOJ, and the BOE keep on going? How might it end?
Yes, it can, although the supply of high quality assets eventually shrinks and causes significant technical problems involving repo, and of course negative interest rates. Remarkably, central banks rebate almost all interest payments to their respective treasuries, creating a situation of money for nothing - issuing debt for free. Central bank "promises" of eventually selling the debt back into the private market are just that - promises/promises that can never be kept. The ultimate end for QE is a maturity extension or perpetual rolling of debt. The Fed is doing that now but the BOJ will be the petri dish example for others to follow, if/ when they extend maturities to perhaps 50 years.