Justices can be impeached and removed from their position, if I remember correctly. This is something that hasn't been done enough, or at all in the past, probably due to political reasons. The mechanism to fix problems in the Supreme Court is there, we just haven't done a good job in making our Congressmen use it when it is necessary. If they're appointed and feel like they can rule on cases without impunity, you will get predictable results when an activist judge is put on the court. The one glaring example I can remember was Bader-Ginsburg ruling on eminent domain citing European/international law instead of the Constitution. I think she should have been removed after this ruling, but, as usual, there were no repercussions.