classical music enjoyed a spike that had not been seen to date.
I don't even begin to think that is true. Leonard Bernstein's Mass outsold SOB just a few years later. If you want something that popularized classical music think Fantasia, not SOB. And what made classical music extremely popular in the modern era owed far more to Luciano Pavarotti and the Three Tenors and Andrea Bocelli (in unit sales THE most popular classical records ever) then the novelty of SOB. In fact, like most pop novelty acts the record sold very well at first, but not much after that. In the same way that Fall-Out Boy sold a pile of records one year, but after that pretty much could not give them away, while records by the Beatles, Stones and Doors sell millions year in and year out, 30 years after their recording. I'm sure that the sales of SOB don't come close to Glenn Gould's Goldberg Variations, or any of several copies of Vivaldi's Four Seasons, or the Deutsche Grammophon catalog of Herbert von Karajan and the Berlin Berlin Philharmonic doing Beethovan's 5th, 6th, 7th and 9th Symphonies.
See classical music sales matter not in 'chart hits' that pop stuff, but in long term catalog sales, which tend to be a very different thing. For example, think of the Stones, the Beatles, and the Doors, yet the top selling 60s artist in that decade was --- Herb Alpert and the Tijuana Brass. And that stuff can't even touch the lifetime sales of someone like Andy Williams (yeah, no accounting for taste, which is what most longhairs say about SOB sales too)
So, interestingly enough you bring up Keith Emmerson, who was among the most dynamic performers when he walked out on the stage and delivered. I saw the Brain Salad Surgery tour (presented live in quad sound) and you couldn't take your eyes off of him, and despite all that, or perhaps because of it, his playing as technique was unequaled at least in rock. While she was a master of laying down tracks and programing her instrument, that does not equate to walking out on that stage and doing it. You can cover up a lot of mistakes in the studio, live you can't hide a single one, and classical music fans tend to really know their music, making them a very hard audience to please.