No big mystery guys,
The "Cadillac ride" resulted from the stability that weight, power, adequate tires, and a long/wide wheel base provides. The last (standard production) heavy cars were the 76's, and speaking from my personal experience, my 76 Coupe is rock solid and smooth. When they started paring the cars down in 77 they reduced the weight and size in incremental steps. At each "downsizing", compromises were made in the handling and ride "envelope" in an attempt to maintain a "luxury" ride. When they got down to the current size platform, there is very little difference in physical dimensions between just about any vehicle.
Just to remind you that a '66 Cadillac weighed in at about 4800 pounds, while the same year Impala was about 3700. You know the difference in ride, and there is very little difference in the design of the suspension components.
The newer you get, the further you get from the "Cadillac Ride".