You could make an argument that its usefulness has decreased the way it is set up right now. Reform seems unlikely as some of the big guys would have to give up on their vetoes. The fact that France and Britain continue to sit permanently in the Security Council yet no one permanently from Africa or South America says everything.
So it's not impossible that some countries will leave frustrated but I think this will be a rare occurrence. Most sober heads will still value diplomatic channels even if they are imperfect.
These international organizations kind of need a world war to reform themselves. WW2 was sort of the end of the League of Nations and the UN took its place. So what we need now is WW3 to get the UN to adapt better to our world today. That sounds great, doesn't it.
I think we only differ in optimism here. You think good leaders can still turn it around. And I doubt it. Either way, I wasn't foretelling the UN's demise.
I appreciate your sentiments in italics and bold text concerning war. I understand that tone is hard to decipher here. If you have taken from my text that I'm pro-war then allow me point out that I am not. I've merely pointed out historical precedence and extrapolated from there. I thought it obvious that the scenario I drew is undesirable. I guess I was wrong, wasn't I.