The absence of term limits means that a president’s ability to select justices is based on the fortuity of when vacancies occur. Jimmy Carter, for example, had no vacancies to fill. By contrast, Richard Nixon got to select four justices in his first two years in office and reshaped the Supreme Court in a way that lasted for generations. Having a vacancy every two years would give all Presidents the chance to equally influence the court. 

Eighteen years is long enough to allow a justice to master the job, but not so long as to risk a court that reflects political choices from decades earlier. A system of government that allows a handful of men and women to hold great power for such an extended period of time is, by nature, more feudal than democratic.

Unlike judicial elections, making the appointment non-renewable helps ensure that a justice won’t decide cases in a way to help ensure continued service. 

Liberals might bemoan that term limits would push a Ruth Bader Ginsburg (who has served since 1993) off the court, while conservatives could lament the loss of an Antonin Scalia (who has served since 1986). But standard terms would treat all justices the same.