The SI system of measurements was created to provide a standard system that could be used and understood by scientists and ordinary people everywhere. No such system had existed before. The SI ('metric') system stands on its own. It doesn't explicitly 'replace' anything, unless the users of other systems decide to throw off their ancient, complicated, and difficult systems and climb aboard the metric bandwagon. To date, every country in the world has done that, except the three backward shadowy societies who prefer to resist change and wallow in their old habits. The three holdouts are Burma, Liberia, and the USA.
In the complex and confusing "customary" system, the "pound" is used both as a unit of mass and as a unit of force.
When "pound" means "poundmass", it can be replaced by the kilogram. And when it means "poundforce", it can be replaced by the Newton.
