The 1980ies had drum brakes as their alternative bike brake tech. For the decades you are talking about, the high end was all about super narrow tires (much mor

The 1980ies had drum brakes as their alternative bike brake tech. For the decades you are talking about, the high end was all about super narrow tires (much more narrow than what you see today) and brake calipers for narrow tires can be built to have very little flex. And that flex is the only difference between a disc brake and a rim brake, if you take a step back and consider the rim a very big disc. For a fat-bike with 100+ mm tires it would be almost impossible to create a reasonably well-working rim brake, due to all the flex happening along the way But for the 20mm tire that defined the entire performance market in those years, caliper flex is a much smaller problem, almost as good as that of a disc brake caliper. Even when mountain bikes took of with their ca 50mm tires it took decades for disc brakes to establish themselves (with quite a few failed starts) and people were happily buying "brake boosters" to reduce the flex (cantilever brakes flex the frame/fork). Road bikes more use tires typically about 50% (or more) wider than in late 20th century, and quality calipers have become a rather tight fit (or wouldn't for at all, for the wider-tired subspecies), but calipers have still improved generation over generation.