The % of corporate debt rated BBB has also exploded. It is likely these companies should really be junk, but are kept alive by reluctant/agreeable rating agencies and historically low rates.

"At the same time, the lowest rated part of the investment-grade market has grown particularly quickly. Globally, BBB debt makes up over 50% of the market versus only 17% in 2001." - https://www.blackrock.com/institutions/en-us/insights/invest...