A couple points:
It appears to me that Zenith was probably one of the last holdouts still building a good, durable set, in the US at least into the late 70s. But by that time, TVs had begun the slide into commodity and consumer's weren't willing to pay the premium.
There of course was the famous (infamous) TV dumping case against the Japanese that was dragged out so long in court (in exchange for Japanese support of US policy) that every US-made TV company except Zenith and RCA was gone by the time it was ruled the Japanese were dumping.
That does not mean that ALL Japanese companies were dumping... There is a small article in a 1977 issue of Consumer Reports called "Why nobody's mad at Sony" Pretty much says that Sony was selling sets on quality, not price, so they weren't part of the dumping case. Naturally, CR takes the opinion that dumping is a "good" thing, because it means lower prices for consumers. (scumbags)
So they outsourced. First to Mexico, then to Asian countries. Quality went in the dumper, and the reputation began to slide. The tried to diversify into computers, but that went nowhere. Enter Gold Star to buy a good chunk of the company.
They pinned their hopes on a compatible HDTV system, as was required by the US government at that time. In the early 90s, under intense lobbying from foreign companies, the FCC's compatible requirement was dropped. Practically overnight, Zenith stock became worthless because nobody needed the patents on a compatible HDTV system.
Gold Star steps in and buys the remains. Their name in the US is junk, so they re-badge it under the Zenith brand. This buys them some time in the US market, and they re-enter as "LG". The LG products are better than the Gold Star, and the Zenith name is phased out.
Right? Wrong?
I also believe that Zenith was an early pioneer of flat-tubes, at least having them as early as '88, and those WERE good tubes, not the junk that came after '93. They did continue with a "rounder" 23" tube in the entry model sets. I remember seeing a lot of those in bars.