In 1972 GM decide to kill all of the marquee's motor programs with the exception of Buick's aluminum V-6, Chevy's small block V-8 and Oldsmobile's 350 gas engine converted over to a Diesel engine. GM also retained the Chevy straight four, the straight 250 six, and the Chevy big block 454 engine for use in all trucks and SUV's, as well as the Olds 330 V-8 gas engine.
So under the hood of the 1976 Cadillac Brougham there was an Olds 330 painted black instead of a 500 cube Cadillac engine painted blue. Under the hood of a the Buick Electra instead of seeing a big Red 455 stump pulling torque monster there was a black painted gutless Chevy 350. Same for the proud owner of the Pontiac GTO. If he bought one expecting to find an aqua colored Poncho 400 motor it was instead replaced by a smog era SBC painted black.
Some people got so mad they sued GM; the rest assumed that Ford did have a better idea. This brainstorm was instigated by Richard C. Gerstenberg CEO at GM who took a golden parachute the very next year. The reason Pontiac sales and Oldsmobile sales fell to the point were GM pulled the plug on the brand was because people finally realized that if they were getting a Chevy under the hood why pay a premium price for a Chevy when a Chevy had a lower base price than either Olds or Pontiac. Why pay two grand for a Pontiac Split Grill? Buick was saved from the axe only because it out sells Chevy around the world. The Chinese buy more Buicks than Americans buy Chevys so that brand is safe.
Saturn folded because the loyal following of Saturn owners who used to have the unique quality control of a car made entirely by robots from the motor to the welded body panels and paint discovered that all of GM's cars were now made the same way to the same quality control point. The Saturn was no longer "Unique"; so why pay more for Chevy by another name.
Shakespeare asked "A rose by any other name would smell as sweet", which is true. But GM discovered no one would pay more for a Chevy called an Olds, or Pontiac or whatever the name plate said. There was nothing to distinguish between the brands any more after 1973. That is the date GM died, not when it went into bankruptcy because they were not bringing in enough money to pay pensioners and executives on the board and the creditors and those few remaining union workers on the line.
By the way the board in the law suit claimed GM had no choice because of emission requirements being enforced they had to loose the big American V-8 engine. The 1973 oil embargo had pretty well sealed the HiPerf engines fate and was it is acknowledged as the death knell of the Muscle Car Era.