“The Russians will stick anything on a T-72 chassis, won’t they?”
It works. They use a proven chasis, where we do a study and tweak things to death to make an all new vehicle ten years later.
We do the same thing, to an extent. Our bridging vehicle was based on the M-60 chassis and the Sgt. York was a rehashed M48. The reason we spend time on new vehicle designs is that we invariably find fatal flaws in older systems and have a robust compete system that chases new requirements.