Listening to Miles Davis, Kind of Blue only confirms my point that 90% of everything is crap; all you have to do is look -- or listen -- to everything else released in 1959.
90% of everything from the 50s, 60s, and early 70s was crap. 90% of everything was crap in the late 70s, 80s, 90s, 2000s, and 2010s, too. 90% of everything will be crap in the 2020s, 2030s, 2040s. 90% of everything was crap in the 1940s, 1930s, and on back as far as you want to go, and 90% of everything will be crap as long as humans keep making stuff.
It's certainly true that the technology and constraints have changed over the years, but that only changes how we humans produce good stuff and crap stuff. It doesn't change the ratio of crap vs good in the slightest. That consistently hovers around 9:1, no matter how or when you make it.