Sure...the gist though is that the central bank responses to any situation with only two tools, interest rate, and money supply. If the situations shows them th

Sure...the gist though is that the central bank responses to any situation with only two tools, interest rate, and money supply. If the situations shows them that there was too little money, they add money. A reason why thy might not have had enough money in the system until QE was that the steady state market equilibrium before the 2008 crash might have genuinely required less cash. There is no law that says that after shocks that the market will always equilibrate to the same place. it is specially the nature of shocks that market conditions are shown to not be in the equilibrium that we assumed they were. You may see signs of one happening, but it is far from clear that you could correctly navigate all policies and market participants to re-equilibriate peacefully.