Question: was stereo the beginning of the downfall of recorded music quality, way before the digital monstrer appeared?
No, mono vs. stereo has nothing to do with quality.
Look at it this way; making a stereo mix of music that was original recorded and mixed in mono is like colorizing a black and white movie. It's foisting a newer technology on an older art. It doesn't improve it, it just makes you wonder why that guy's vest, that woman's dress and the drapes are all exactly the same weird shade of green...
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The article is good, but a couple things - the first run of the mono box is not numbered, and this; "they would often not include their hit singles in their studio albums (feeling that would be ripping off their fans since they had already bought those singles)," is incorrect.
Singles were never part of albums in those days. They were looked at as two different things. There were probably marketing reasons for that - "The kids won't buy the album if they already have the songs!" - not to mention the fact that most kids could afford to buy singles more than albums, but it was not something specific to The Beatles. If they were really concerned about "ripping off" their fans, you would not have seen the thousands of Beatle products for sale at the height of their popularity. Not that they directly profited from all of that, but the certainly profited from a lot of it.
Later that attitude toward singles would change in a lot of places, as albums became collections of singles, and singles were pulled from albums. When the Wailers recorded and released their first proper album in Jamaica a lot of people were confused, thinking they had somehow missed a year's worth of singles. ;)