Related story - my first job out of school was in investment banking. My desk worked on some esoteric securitization products (basically bonds backed by aircraft and shipping container leases) where all issuance had basically disappeared when I started, which was right after the 2008-9 crisis. These products generally were in the A/BBB area, and generally had traded like high yield bonds before the crisis. When I first started, we struggled to find investors and were generally seeing 6-8% yields on some small deals. By the time I left three years later, yields were getting down to the 4% area, issuance sizes had tripled and new paper was routinely 3-4x oversubscribed. I have some friends who still work there and tell me not only have yields kept coming down, but lower quality leases are being thrown into securitization pools. I 100% agree on all the comments here saying the big story is lower rates driving people into riskier investments. When the next crisis hits, people will talk about how negative rates forced people to reach for junk companies and questionable securitization paper.