In an alternate world, where congress and unelected regulators didn't throw themselves into product development, the B-platform cars would have major restyles and refinements every 3 years. You can thank the Carter administration (and the fact that government never downsizes) for that. With CAFE standards going from 18 MPG (1978) to 27 MPG by 1984 (9 MPG in 6 years!) there was:
a) Precious little time to do anything but downsize using known technologies (weight reduction).
b) Precious little budget left to update cars that were supposed to disappear by 1985.
c) Enormous demands on engineering to concurrently develop cleaner and safer vehicles.
d) An incoming Republican leader less inclined to partner on R&D.
e) An entire world of competitor nations who had no problem lending (or giving) money at 0% to promote exports for industry, while keeping home markets free from import competition.
In other words, absent or with fewer of these concerns, you probably would have seen the same 1980 updates. But by 1983 you likely would have seen an entirely new body/interior on that platform and greater styling differences between the brands than 1977's crash program allowed. And you certainly wouldn't have seen the downsizing-disaster of 1985 because no one would have been shooting for such drastic MPG gains across the board.
One other cruelty of CAFE was its application by "corporation" and sales volume. In other words, a full-line company like any of the Detroit 3 needed that average economy number across millions of vehicles. So a Chevette is averaged in with a Cadillac because they come from the same company and because of GM's sales volume. Yet a company like Mercedes-Benz imported just a few hundred thousand vehicles annually (virtually all of them in the hi-profit luxury segment) and wasn't thus subject to all of the CAFE mandates. Oh, they made higher MPG versions for Germany, but they weren't sold in the US. Surely because their limited power and cheaper-feel would have harmed the brand's luxury reputation. But for you Cadillac? Your parent company sells a sh*t-ton of cars, so you need some 25 MPG Caddies in the product mix. Enter the V-4-6-8, the HT4100 and the Cimarron. What do you think that did to brand equity?
Keep in mind, Project 77, along with Ford's Fox/Panther platforms and even Chrysler's R-body re-shuffle were in the works when CAFE was just a place to get lunch in France, so it would appear that markets do, in fact, work. Unless artificially distorted.
