A couple of month's ago, the PCA magazine Panorama published an interview with Manfred Jantke, Porsche director of
PR and Racing during the '70s and '80s.
He said the original 928 was developed largely because Porsche was terrified of the effect Ralph Nader's book "Unsafe at Any Speed" was having on the American market for rear-engine cars.
Jantke also said the same concerns led to the development of the Targa top, i.e., fear that Nader's crusade against convertibles because of safety issues would lead to anti-convertible legislation.
As it turned out, the air went out of Uncle Ralph's balloon once he had killed the Corvair, but convertibles did become very scarce for about 20 years. Porsche did not take the plunge back into the convertible market until '83.