The US has spent billions -- easily hundreds of billions, if not trillions -- on its air defense. From F-16s to F-15s and Patriot missiles, all branches of the armed forces have been active in ensuring that America owns the skies. So it's absolutely unthinkable that, on a given Tuesday morning in September, the nation could find itself without at least a squadron worth of Fighting Falcons or Eagles, armed with AMRAAMs and Sidewinders and ready to shoot down a hijacked airliner bent on causing terror on a massive scale. That's why it's difficult to imagine the 9/11 attacks being successful. The flights that originated in Boston were intercepted by F-15s from Otis AFB on Cape Cod less than 15 minutes after evidence of hijacking emerged.
It can't possibly have made a negative impact that NORAD staff were simulating a mass hijacking on the day the events became "real world." As Nena sang in "99 Red Balloons", "this is what we've waited for/this is it, boys, this is war." The very situation they had practiced so many times suddenly played out on the skies above Massachusetts, New York, New Jersey, Pennsylvania...
Could the attacks have succeeded? Perhaps, if you deepen Clinton's "peace dividend" cuts to the military so much that the world's greatest air force could muster, let's say, a couple of unarmed F-16s on the Day of Judgement. Even if that happened, we've got to consider the possibility of the hijacked passengers fighting back. The Washington, DC targets (if we are right to believe the 9/11 Commission's report) may have had Stinger missiles on hand, as well. So, the worst possible 9/11 I can imagine would be the impact of two planes into the World Trade Center. We have no way of knowing what result that might have had, although the building's designers have claimed the WTC was designed to withstand an impact of a Boeing jetliner somewhat smaller than the actual hijacked aircraft. Some engineers have said the lightweight structure of the WTC, based on floor trusses connecting a concrete core with a steel exterior, might have been vulnerable to fire, but, again, besides a few tests carried out by NIST, there's no real evidence to suggest the WTC's sprinklers and fireproofing would have failed. The buildings would, of course, be seriously damaged, and might even require demolition, but they would have remained standing long enough for emergency services to rescue the vast majority of the WTC's inhabitants.
And what if we go further than the worst case scenario -- what if we're wrong, and the WTC is destroyed? The political response would be what we saw, but on a larger scale. Massive Tomahawk and B-2 strikes against targets in Afghanistan, leading to the eventual collapse of the Taliban. Boots on the ground is a possibility, but I suspect it would be mostly special forces on missions to eliminate specific Al Qaeda leaders. The Soviet experience in Afghanistan was enough to demonstrate that that's not a country suitable for long-term military activity.