I would argue that it's not practical to burn the current system down without a plan at all for the next system (like the ACA a few years ago. . .)
My concern isn't change. My concern is the complete lack of concern for consequences. Like it or not, the US is and has been on the decline in terms of world authority. Leaving a power vacuum, like dismantling the UN, will just open the door for places like China to step in. You think that country has any amount of give a crap about humanity as a whole? Not even a little, I would argue.
So, again, for many people, it's not that the UN is perfect (or even functional in my opinion). It's that there is, has been, and seems like there will never be an actual plan. Am I wrong?