People opposed to the Iraq war thought that we should have kept the troops in Afghanistan to finish off Al Queada rather than redeploy them to Iraq. Perhaps they are willing to acknowledge that this too was a bad argument.
Moreover, assuming that Iran was as responsible for 9/11 as Afghanistan, but that it is a much tougher nut to crack, perhaps it makes strategic sense to have US troops on both its western and eastern borders before hostilities commence. And, if you ask, why should Saddam have to suffer for the sins of the Iranians, my answer is he deserved to be removed anyway.
If taking out the Taliban was legitimate, what about attacking Iran? If the argument is a pragmatic one about US troop strength, perhaps simply making Iran a no fly zone might be sufficiently destabilizing to allow the students to take over. Perhaps in combination with killing revolutionary guards attempting to attack the students…
Some thoughts on attacking Iran:
Michael Ledeen thinks we should attack.
CFR thinks we should talk to them more. So does Gregory Djerejian but see Ledeen in the comment section! But even Greg thinks we need to do something and so does Amir Taheri.