Coordinated attacks have always been possible on US soil and have been made even more so by the response to 9/11 (e.g. big group of people piled together in front of every security line). It's not a "very, very dangerous thing". There is always something that is vulnerable, and yet no one has exploited this yet. Has this world-wide cabal of highly advanced terrorists someone missed this vulnerability that I noticed the first time I flew after 9/11?

India has a history of terrorist attacks from islamic extremists (largely originating in pakistan). The US has no such history.

A Mumbai-style attack will always be possible, on any soil. It's not possible to secure entire cities of millions of people against a handful of people with rifles and grenades.