9-11 didn't cause US decline:

Was 9/15 a greater game changer than 9/11? Many believe that the collapse of the World Trade Towers on 9-11, 2001 changed the world. Looking at the numbers after the collapse of Lehman Brothers on 9-15, 2008 suggests it was. After 9-11 the US global political and economic dominance re-surged. The financial crisis sparked by Lehman’s insolvency however, forced our nation to acknowledge our limits. The rising economies of China and other rival nations while the US economy is drained by an endless war against an extremist group who’s only real hope is to break us economically and divide us politically is progressing faster than they had ever dreamed of. 9-11 and a small number of extremists the world over didn’t cost us our future. Our reliance on a greed inspired unregulated economic system and waging an endless war against a tactic did.