There are boring institutional investors and fancy equity investors who buy treasury bonds. The latter do it because they want dry powder during a recession. QE takes treasury bonds out of the system without issuing more bonds, meanwhile banks get central bank reserves that they don't want. Nobody is borrowing, no money is issued. Everyone is worse off. As yields go to 0%, those institutional investors simply hold cash in bank accounts which drags the velocity of money down.
The unfortunate news is that most macroeconomic theories (including mine), especially the neoclassical variety, are kind of useless in the real world because of inability to actually implement policies 1:1 in the real world. The modeling errors are also quite significant. Nobody is really trying to solve NRAs(non reproducible assets) or monopolies. Everyone just assumes you can harvest more bananas or colonize a second planet.