A quick reply about my quad encoding from back in the day. The decisions about quad releases were ultimately deals made between the labels and the marketing divisions of the hifi manufacturers. In my case it was a cooperative agreement between the label’s parent company and Sansui cross promoting their QS standard and its hardware while supporting some quad releases from the label.
Those of us on the creative side got the opportunity to produce quad mixes. For me, though, that ended after one album when the deal between the label and Sansui expired. No deal, no budget money to mix or release in quad. It was the end of the quad boom and neither corporation saw much of a future return on paying for more quad releases. So, back to stereo-only. That's the economic reality of the record business.
However, my story isn’t over yet. There are multichannel chapters still to be written. (Taking a little longer to execute than expected).