You talk a lot of sense, the trajectory of the decline, mostly, I agree with you.
Sadly your insensate Seville / Nova comparisons are simply, technically, wrong. The frame on the Seville is unique and is much closer to a Camaro than a Nova. Much of the front suspension, engine compartment and rear axle, Camaro. I've been with the '76/'79 Seville over 45 years, I now have a Alternative Parts Book developed, almost complete. This explains where the Seville parts come from for restoration purposes, as well of course for service parts requirements.
The Oldsmobile engine, not standard, but built to Cadillac's higher standards and checked and tested at Clark Street to Cadillac's own high engine tolerances.
The sound proofing? They used, for the first time we believe acoustic isolator material, meaning specific sound absorbing materials in specific areas in order to kill the noises from each area of the body.
Regarding your 'reinforced' comment? How about the use of Fast Fourier Analysis and Nastran computer programmes that had been used by NASA for the Apollo Moon missions? It's rather more than reinforcement. There are several articles, one in '75 in Automotive News about this technological leap in terms of body development and structural analysis.
That, you say, someone has built a Seville using Nova parts is incredible, most simply won't fit properly, certainly caster, camber and toe-in measurements would be all over the place? I run a '78 Elegante that's had a long life and is still a daily-driver, I also run a '79 Rolls-Royce and it has to be said, the Seville is a much better road car and is far quieter (decibel tested) than the Rolls. Sure there are elements of a Rolls that takes 12 weeks to build that are stunning, but that's not what we are discussing here. The technology used to get the much more basic platform of the Camaro/Nova put the Seville right at the very top of luxury cars in the day.
I agree completely that Cadillac read the Seville's success wrongly, hence the cynical Cimarron and the fact that here in Europe, especially Germany, they could've taken on Mercedes on their own patch. GM just missed that entirely. So sad.