Originally, the Supreme Court was just supposed to be the highest court. The Founding Fathers wanted those positions to be apolitical, and they thought a good way of doing that would be making the position last a lifetime. I.e., if you dont have to worry about getting re-elected, your decisions will be more pure.

Things changed due to Marbury vs. Madison, where the court gave itself the power of judicial review, essentially the power to change laws, which is where they became much more powerful. Nowadays, people are taught that there are 3 branches of government with checks and balances over each other, which is true, but it was not originally that way until Marbury vs. Madison.

The problem with literally any government form, though, is that there is never any immunity to bad actors. Term limits dont necessarily solve anything, but if the justices are acting like any other politician anyway, we might as well treat them that way.

I would argue the best solution is ironclad ethics rules that are grounds for impeachment. E.g., if you are clearly owned by a billionaire, you should be impeached. That comes back around to the “no safety from bad actors”-problem, though, when congress refuses to do the “right” thing.