I disagree entirely.

Anytime you make the body a variable in the suspension equation, you have the opportunity for inconsistent application of rate and control. Additionally, in the era these cars were originally built, ride comfort was a much higher priority than it is now. This flex was deemed acceptable to teh factory engineers based on the rates being used in suspensions back then. As drivers have demanded more from the chassis, designs have grown more rigid to accommodate the higher wheel rates. To keep suite, older cars have to have that rigidity added to them.

Similar improvements in rigidity can be achieved by doing seam welding, but this is a very labor intensive effort that most don't want to do and is not easily applied to cars not getting complete body and paint makeovers.