I had almost forgotten about the torque tube with the curved drive shaft inside . A unique design .
I suspect that the scarcity of that model was due to metal fatigue failure of that component as the years went by , and ultimately , unavailability of that component as a replacement part . Every revolution of the drivetrain flexed the shaft considerably , and I believe failures were common .
In the late 70s , 80s and 90s I got turned off on Cadillac because of motor failures . I had a beautiful ’79 Seville with the Olds diesel engine . I had to put a new motor in it after it blew a head gasket and that destroyed the engine . Later my wife had two Cadillacs with the split block design ( the engine was cast in two identical halves , four cylinders each , which were reversed and joined before final line boring of the crankshaft journals ) – both of those vehicles had to be sold ( at a huge loss ) when they began exhibiting imminent motor failure . They both were serviced regularly at the Cad dealer , so the failures were not due to lack of maintenance . I suspect that both the diesel issues and the split block problems could be traced back to beancounters and marketing pushing to get novel designs to market before all the bugs were worked out . The diesel was based on the 350 gasoline engine design and not strong enough for the pounding of diesel combustion .