What have we learned?
For those who took the time to craft top 20 lists for all the decades, did any revelations occur? Or truths that should have seemed self-evident but for whatever reason weren't until you did this?
For me, I learned that the four genres I've half-suspected are my four favorite genres are indeed, if the numbers tell the truth. I also learned that each of those four dominated a certain decade. Observe the pluralities and majorities.
14/20 of my 1970s list was progressive rock.
8/20 of my 1980s list was traditional heavy metal (with some on the fence ones as well: Queensryche, Gn'R, Helloween)
5/20 of my 1990s list was death metal (with Opeth on that fence)
8/20 of my 2000s list was progressive metal (Nevermore and Porcupine Tree on the fence)
I also learned that I don't really care for the 1960s all that much (not revelatory).
So, what have we learned?
Note: A perfectly reasonable answer is "nothing".
Mortals are mortar and life is the fuse.