Ever actually read the Constitution? They are not appointed for life in the Constitution, any more than separation of church and state is in the Constitution.
They are appointed and “shall hold their Offices during good Behavior”. Can you imagine the chaos if each new POTUS could fire the Federal Judiciary at whim?
SCOTUS judges are appointed as long as they are on “good behavior.” As far as I and many others are concerned, repeatedly ignoring or changing the Constitution from the bench is NOT good behavior and should subject the justice/judge to impeachment