And yet after it the votes changed and they allowed the New Deal. Courts become less extreme when their comfortable power is threatened.
What’s your solution to a corrupt court throwing away precedent and making law from the bench? Just pat Mitch McConnel on the back and say “shucks, you got us Mitch, guess we’ll just live the rest of our lives under conservative rule”? Because waiting for 67 Democratic senators or multiple conservative justices dying under Democratic rule isn’t likely to happen.
Adding more justices may instigate a tit-for-tat, but it’s no worse than just accepting that they get to make law for the rest of your life, and the credible threat of doing it (or the actual practice) is likely to lead to real functional reform.