You're ignoring the reverse causality. Antitrust lawsuits against Microsoft in the 1990s/2000s put them on edge, and made them think twice about strong-arming their competition. Back when Google was starting to make a name for themselves, MS strongly considered adding a warning on Internet Explorer, telling people to "beware" of any results they see on Google. MS eventually decided against it, because of the antitrust magnifying glass they were under. Having a level playing field allowed Google to grow exponentially, and eventually rendered MS' monopoly irrelevant.
Monopolies use anticompetitive tactics to preserve their moat, and continue being monopolies. When antitrust legislation works effectively, this moat disappears, and the monopolist is eventually overrun and becomes irrelevant.