I've commented on this stuff before. I used to work at GRT back in the day. For most of you collectors, they are the medium blue cases and labels for house brand. We also made tons of recordings for ABC, RCA, and most of the other labels on contract. GRT made enough money off the 8-Track business to buy Chess/Janus Records and their whole library. We also made R2R tapes and Cassettes by the truck load. By the way, 90% of the bulk tape was BASF. The rest was Scotch. If you think about it, they are still playing today and most have not shed - pretty good binder !!
The deal with Mr Lears design was to get away from the 4-track car decks. The thing about the 4-track car decks was that anyone with a 4 track reel machine could make boot legs, and they did. Same 1/4" tape, same track alignment, still center spooling tape, but only one track change on the splice or it could be done manually at random.
So, Mr Lear came up with the 8-Track. That put 90% of the bootleggers out of business as they had no source of duplicating machines. Ampex made most of the duplicating slaves back then and they would not sell to anyone who was not a legit business. Eventually Eagle (Tx) got into the act making high speed slaves, but they sold far more cassette slave decks than 8-Track. Anyway, it used 1/2 the raw tape and about the same case cost. That lead to production cost cuts in terms of time and materials (twice as much music on each slave pancake on each run).
Anyway, we regularly QC'd 8-Tracks against the other formats. They were about on par with commercial cassettes and a bit behind commercial R2R (@7.5 IPS). You got to remember where these other formats were - only Dolby B, no DBX, no HX-Pro, nada.
In that world, 8-Track holds it's own. But 8-Track died because the format was bulky and the cassette guys just kept making their gear better and better and no one was going to do that for 8-track. There are good 8-tracks out there (cartridges and home machines). Before I pitched all of mine years ago, I had some Robin Trower like Bridge of Sighs on a green label 8-track that was pretty stunning. We didn't do that run, but we did a LOT of Rolling Stones and their QA requirements were pretty tight. It can work, you just need the right pieces