That's an amazing list. Thanks so much for pulling it together. I'd argue that the explanation for the outpouring includes everything you've suggested but goes much broader and deeper.

Like all popular music, rock is a both a product of and a commentary on its times. So if you want to understand why 1970 was such a remarkable year, you have to look at what was going on in society when the artists would have started thinking about and writing the music for those albums. I'm guessing that they'd have begun doing that at least a year, more likely two, before the albums came out: or around 1968, the year of shattering events such the assassinations of MLK and RFK, the Vietnam War, race rioting nationwide, and much more.

In other words, I suspect that the artists were responding on some level to those events as much as to anything that was going on in music, the music business, or their own careers.