The SCOTUS is not unchecked. The Court's time-frame (many decades) is necessarily -- and by careful design -- much longer than those of the Executive and Congress (6 years or less). Justices are placed on the Court for life, but all eventually retire or die.
It is only our (understandable) impatience that causes us to not see the Founders' wisdom in having one of the three branches work in very long periods.
I don't follow your argument re life tenure for SC justices.
The purpose of life tenure was to insulate the justices from political pressures so they could make proper, sober legal decisions. It was not to give them license to act beyond the scope of their constitutional authority or to "interpret" the Constitution and federal law according to their personal political preferences. The Framers did not intend for bad decisions to go unchecked for decades, until "better" justices could be appointed to the Court.
The great flaw in the constitutional scheme adopted by the Framers is precisely the lack of any meaningful institutional check on the Supreme Court's behavior. I believe James Madison proposed some sort of council of review, but I'm not sure if I recall that correctly. In any event, we have evolved a system of judicial supremacy that clearly is contrary to what the Framers intended and to what the American people would vote for if given a chance today.