True, there is a minimal rate of unemployment due to people switching jobs. But that's different from "natural rate of unemployment", which is more an argument

True, there is a minimal rate of unemployment due to people switching jobs. But that's different from "natural rate of unemployment", which is more an argument used to prop up unemployment. Once everyone has a sufficiently decent job, workers will have have enormous bargaining power, because they do not fear losing their jobs. Imagine how negotiations between bosses and workers would go. The ability to openly disrespect the boss, and the clearer possibility to transcend boss rule. Absent other methods of control (like the USSR used, as you point out), capitalism would risk overthrow; that is, replaced by another system, with different defining institutions. Needless to say, this poses a problem for policymakers, whose success depends on their ability to serve the wealthy elites who finance their campaigns and dominate government. So they must pursue economic policies which lead to sufficiently high unemployment (often sacrificing growth too; obviously less people are making stuff); or at least fragile employment, like in the service industry.