I think the main thing that can be said to defend keeping the Constitution is simply that it is a Schelling point. We need some way to base our system of laws. What system do you choose? There are arguments for many options, and I'm not saying the Constitution is necessarily the best. But due to what you may perhaps call a historical accident, the Constitution is where we are now. This makes it a Schelling point for all the different options for a system to base our laws on.