Pontiac was forced to add the EGR by the emissions mandates issued by the EPA.
Since using an EGR valve was the ONLY known solution that would accomplish the lower emissions levels at that time, the EPA basically forced ALL automakers to adopt the technology.
The EPA probably didn't send them each a letter saying "You will have to use an EGR valve", but the requirements they set basically did that for them.
My point was that, given the amount of extra heat generated in the intake by the EGR valve, Pontiac fell back on the durability of a cast iron intake for the production SD455 engine.
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