Competent person in charge? No. 9/11 is mainly the fault of the Bush administration's incompetence. That by the way is the reason, why they have tried to prevent proper analysis of what went wrong.
As well there are quite a number of security issues, which were improved, since. Security is expensive and when for a long time nothing happens, it is attractive to let the level of vigilance dropping. 9/11 has reversed that. Survaillance, sky marshalls, and as well the crackdown of the financial streams is much sharper than before.

What if we pull out of Afghanistan and the Taliban regains control, allows Al Qaeda to set up its training camps again and continues to plot.
Well, this can happen. But what happens, when they plot in the tribal areas of Pakistan, or in Somalia, or in France or in the USA?

The terrorists don't need a major training camp to pursue attacks in the developed world. Plotting they can do everywhere in the world. But even when they needed them, they have plenty of options. The US can't invade all countries who harbour a couple of terrorists. For the terrorists moving from Afghanistan to another country is a burden, but doesn't finish them off. For the west to make a meaningful nationbuilding, which guarantees, that the terrorists can't come back, once the western troops pull out, is quite effortful.
I think even just invading the tribal areas of Pakistan will be very dangerous, and getting the Pakistani army to do it themselves is difficult and associated with risks, too.

Furthermore the costs of an attack like 9/11 as well in human live and pain, are not infinite higher than the cost of a war. If you can reduce the risk of an attack like 9/11 by 5% (which I very much doubt) by 'winning' in Afghanitan, but you have to sacrifice 5,000 soldiers and 50,000 seriously injured ones, it is not worth it.This doesn't include a single victim of 'the other side', which will go to the 100,000, until Afghanitan is won. You have to accept, that their live has some value, too, even when most of them are 'regular' targets, but of course there are a lot of purely civilian victims, too.
In terms of money, it is of course even more so. The war in Iraq (which could indicate how much effort is needed to secure Afganistan; just because there were really terrorists in Afghanistan doesn't mean it is easier to win in Afghanistan than in Iraq) costs probably each year more than 9/11.

except of course not acting like assholes
But that is a quite important point. And the west is losing the propaganda war in middle east. Not only the war in Iraq, but as well the war in Afghanistan are seen by a number of relevant people as occupation of a muslim country by unbelievers. This can increase the risk of a terror attack. The most idiotic thing is of course the security support of the Saudi monarchy. That probably is the single most important point. Second would be the Israel/Palestinian conflict. Both sides are strongly supported financially by the west. We could say, either you both make bold moves to peace, or we pull out our support. Or not, it is really only the second most important issue.

And finally, war is creating chaos. Chaos creates unpredictable outcomes. So far in the argumentation it is assumed, that Afghanistan is won, if the US gov is just determined enough and providing sufficient resources. That may be a flawed assumption. There can happen unpredictable things, which could prevent the US from winning in Afghanistan, even when fully determined.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers