Prior to the 1980s, monopolies were considered bad (or at least extremely dangerous and suspicious) by default, and leveraging one to privilege your entry into another market was a huge no-no (though I'm not sure leveraging a monopoly to create a monopolized adjacent market would always have been seen the same way).

Robert Bork's theory of "consumer harm" as the guiding light of anti-trust enforcement (and further conceiving of "harm" mostly in the form of higher prices, or at least "value for money") has given corporations too much wiggle room and consequently led to a lot of contorted legal and economic arguments, substantially reducing antitrust scrutiny and enforcement (which was the whole point, AFAICT).