Earlier quoted context omitted.
Perhaps this is a good reason to have some sort of technocracy element to the federal government. I lean extremely far right, but in this case, this seems like one of the few things that the federal government should be doing-- breaking up stout monopolies that can't be competed with. I'm unfamiliar with the decision, but why split up a company like Microsoft in 1999, but leave Facebook and Google alone?
It's not against the law to have a monopoly - it's against the law to exploit having a monopoly to enter another market. Microsoft was seen to be exploiting it's consumer OS market share to gain a monopoly in the browser market and the productivity software market, AIUI.
It is against the law to acquire, or to perpetuate, a monopoly by any combination or conspiracy in restraint of trade.
Although you're quite right that the move into the browser market was a big part of the case against Microsoft, there were other pieces to that case. There was a whole bunch of work around APIs/ABIs and in particular, denying other parties access to secret or privileged APIs in order to cripple potential challengers to the existing OS market (e.g. Java/Sun).
(Notwithstanding that the Microsoft strategic work around the browser stuff was a very correct reading that the browser was destined to become the de facto OS.)
Yes, it's complicated and there's a whole century+ of interesting jurisprudence. But it's not sufficient to just declare it's OK to have a monopoly -- you can be at risk of antitrust suits even just 1. having a de facto monopoly and 2. doing the "normal" smart business things to hang onto it.
For further reading, start with https://www.justice.gov/atr/us-v-microsoft-courts-findings-f...