None, because as they finally pulled back on QE in 2019 the yields inverted and the overnights jumped, so Trump forced them to dive back in around Sept '19 to keep the gravy-train trucking for the election year bragging rights.

I'm looking at different times scales. There have been pops every 7-10 years or so and they get worse every time. That is part of the business cycle. Sure, things recover (in huge part do to QE), but that doesn't mean bubbles do not happen on a pet frequent basis?