Manufacturers past their heyday

One of my father's friends was Rudy Bozak. In the late 1950's, he made my dad a custom pair of Bozak speakers. We lived in Connecticut close to where (Dr.) Bozak had a facility - he was a great fellow, and I do not think that his work was archaic during the time in which he lived, but I am not sure whether his brand lived on in any manner.

S.A.E., Harmon-Kardon (I guess Harmon lives on as a conglomerate but the celebrated “Citation” line is a relic of the past.), 

This sounds more like a marketing question. There are fewer high end audio stores that we can just walk into and listen to gear. Then the ones that are open have their favorite brands they carry. These brands have to be ones that bring people in and ones that they can sell at a margin that will pay the bills and they need to make a living selling them. Just as your question is positioned as companies that have fallen from where people used to perceive them to be I will say that there are companies out there that have been hyped way above what their quality really is. This is marketing 101 and marketing cost money. Some companies are content to let their product speak for it self.

Many if not most successful mfg are not in there HEY day.  Those days are reserved for the newer mfgs who come out with a new piece that punches above the others in their class. CJ as others have been doing that for 40 years. It's not unusual for them. The new kid in town always gets noticed. The longevity of their success depends on how long they continue to perform at the level which got them noticed in the first place. But at some point a new kid comes along and gets attention for a product that rivals the former new kid, to which people say, HEY have you heard about this new product? The HEY day starts for another.

Hey there.  "Heyday" is all one word: the time when a person or thing's success or popularity is greatest. I rather doubt even CJ has been at its greatest for the whole 40 years.