Ford fitted what was required for a road car James, with a designed life, this was then improved with greater shore hardness rubber for the cosworth versions, and with all rubber components they degrade over time.

It's not just Ford, Ferrari have the same problem with rubber in its suspension even with recent models, In Fact Ford went out of there way to improve the cosworth versions further still by using spherical bearings in the arms rather than just solid rubber of the lesser spec Sierra. People are actually making their suspension worse on cosworths by then swapping these out for poly bushes, another example of doing something for the sake of it rather than understanding the engineering behind it.