I should mention I base this on the experience of my family. Back then, we were 100% GM - primarily Pontiac and Cadillac.

Between my Dad, Grandfather and Uncle, they owned a string of GTO/Lemans, full size Pontiac Bonnevilles and Safaris (station wagon) and Coupe DeVilles. This had been the case since the mid-50s in my family.

Most of these were redesigned in '71/'72/'73 and stayed in production until '75/'76 with minor changes. The '73s were basically the same quality as late 60s up to that point, in '74 we started to notice severe rusting (NE Ohio) in odd places - like the middle of the roof or the top of the fenders/quarters - we were used to wheel wells and rockers rusting due to the salt, but this was just weird.

Also, a lot of the interior parts, even when carried over from the early 70s, started being made out of cheaper materials and were warping and cracking prematurely, and just felt cheaper. The chromed metal started being replaced with crappier metal or plastic that peeled and discolored prematurely. The engines were detuned and contained more and more hoses and mechanisms to try to meet emissions standards, making them difficult to maintain and almost impossible to troubleshoot and repair. Despite my family having decades of experience with these vehicles, they were often stymied trying to get them to run well - half of them were mechanics either amateur or professional.

Most of these vehicles got redesigned around '76/'77/'78 - sometimes twice. The replacements were a bit better in some ways after the initial teething pains, but then we started having transmissions grenade - even behind V6 models. Lots of quality control issues with drivetrain parts not up to the demands of the vehicles. Suspension components completely rusting out along with the subframes, And the sheet metal got thinner and more prone to rust. The interior cheapness continued, although it was better concealed. Vinyl, which used to last the life of the car in many cases, was replaced by velour which started to wear off in a few years and looked like crap.

By the early 80s, it was a total shit show. The switch to transverse engines/transmissions was a nightmare and they were terribly unreliable, difficult to work on, and it was obvious they were under-engineered. The switch to water-based paints was a disaster with clearcoat peeling off in sheets and then rusting where the basecoat was exposed - and you couldn't keep the clear shiny where it stayed attached.