Well, Justices in my experience are very logical people, and have studied history and the law deeply. It is not perfect, but they try hard to get it right. That is why the president choosing a chief justice is far more important than just about anything else to our every day lives.
And besides, your argument is circular, because existing laws were put there by human beings in government. So they were "move the goal posts" when they were being put in place.
That the Constitution can be amended is apriori acceptance that laws evolve. But it is certainly possible to de-evolve it as well.