It is more serious if large countries withdraw from existing agreements, prevent the rule-based institutions from working efficiently or go solo and test the limits of or act in direct contravention of the written or unwritten rules of the institutions. Or if medium-sized and small countries individually get the notion that they will be able to decide more themselves by withdrawing from the institutions.
In this case they face a difficult choice. Either they must follow the rules laid down by others – without having any influence on them. Or they must accept the loss of prosperity and the hassle imposed on the government, businesses and citizens from opting for isolation and their own rules, plus harvest possible windfalls that may come in the form of bilateral agreements.