They went after MS for LEVERAGING their monopoly to gain access to other markets (specifically the browser). Being a monopoly is NOT illegal. Using that monopoly advantage in certain ways is. And of course, MS is still in the browser market. So you see just how effective that was. And of course Google doesn’t have anywhere near the market penetration MS does. These days people confuse big with monopoly. The very fact that people insist 3 different companies that compete with each other in various markets are all monopolies shows just how silly the word has gotten.
And it’s possible for non monopoly companies to engage in illegal competitive practices etc. But would it be fair to call mobile OS market a duopoly? Google and Apple have like 97% of the market between them. Not that duopoly is any sort of legally actionable category.
Let’s be honest, politicians went after microsoft because microsoft declined to get financially involved in politics back then.