If legitimacy does not originate in the means by which power is gained, it must instead accrete from the ends toward which power is employed. Thus, while the monopoly on force is never legitimate in origin, since it is always based on an exercise of arbitrary force, it can become legitimate via governance: by transforming from a regime that disproportionately benefits the rulers at the expense of the ruled into one that trends toward equality. As a corollary, a government that has obtained legitimacy may lose it in time.
Of course, the point at which legitimacy is gained or lost is devilishly difficult to determine. If one gang protects your community from other gangs but demands half of your livelihood in exchange for not destroying you, is this gang’s rule legitimate? This entails a grave risk-reward analysis: is your community’s vulnerability to other gangs, and to this one, worth giving up half of everything? More relevantly, if the U.S. government can be considered to have achieved and maintained some modest degree of legitimacy, but Trump and the Republicans in power proceed to turn it into even more of a self-serving power brokerage at public expense than it already is, will the American government remain legitimate?
That depends not only how inequitable the rule becomes but also on its degree of entrenchment. If something sufficiently closer to equity can be restored within the system, i.e., through the constitutionally prescribed election process, the rule may still be considered legitimate. But if the rule succeeds in entrenching an extreme degree of disproportionate power, whether by blatant force or subtle manipulation, its legitimacy, and therefore its claim to a monopoly on force, may be questioned.