I think Pink Floyd's
Dark Side of the Moon is a watershed.
If you listen to the bootlegs of Floyd playing some of that material before they went to the studio, it's much better. It's got that pre-Dark Side feel: genuinely transcendental collective-ecstatic psychedelic music. But then they went into a studio and spent a hundred years overproducing this plastic soul-less hyper-romantic studio fantasia we've all more-or-less had to endure since. Moreover, Dark Side set us up for the ever expanding
scale of rock music. A lot of good musicians got out around that time or before (Fripp, Gong) because they saw where it was going: "concerts" in which several million gather to "listen" to a "band" "playing" some miles away. Dark Side was the foretelling of the ghastly aesthetic monstrosity that culminated in The Wall. That led to Pink Floyd being rightly and properly lampooned in HHGTTG...
Take the grotesque scale of Wagner's conception for opera at Bayreuth, keep the theatrical romanticism, give it vast budgets and modern electronic technology, subtract most of the musical innovation and you've got mid 70s Floyd. That's the watershed Dark Side ushered us into.