Unfortunately much of this has been due to the rapid decline of the dollar and the Fed still buying market backed bonds (buying their own debt)
Technically, the Fed has been buying bonds, but effectively only rolling over (buying new bonds as old ones mature) since 2014, not adding to their positions. Yesterday they announced that they will stop rolling over next month.
Now the question is if other central banks will step in to take their place, or if assets will move to reasonable valuations.