When the Constitution was originally framed, life expectancy for the average American was about 40 years, or less. Our founding fathers did not imagine a world where a judge would spend 20 years, or longer, on the Supreme Court.
Further, since federal judges are not elected and cannot be recalled, the American people have no recourse when they are dissatisfied with the court system or a specific judge.
Federal courts are made stagnant by lifetime appointments.
A 4- or 8-year term limit per federal circuit/district, and an 8- or 12-year term limit for the Supreme Court, would allow a greater turnover and more diversity in the courts, and would also help relieve Congressional deadlock over judicial appointments-due to their relative impermanence.