The danger of a monopoly lies in its ability to exploit a market, not in its longevity.
Markets can change in ways that destroy monopolies. For example, if in 1900 you had a monopoly on horse-drawn carriages, that wouldn't have prevented you from being put out of business by the advent of the automobile.
Markets can change in ways that destroy monopolies. For example, if in 1900 you had a monopoly on horse-drawn carriages, that wouldn't have prevented you from being put out of business by the advent of the automobile.