Sending Predator drones there is a very poor way of incapacitating their government (they are useless without HUMINT on the ground) and it's foolish to use these drones since they can easily be taken out by Iranian airforce, not to mention it will provoke Iranian retaliation against U.S. interests.
I think you underestimate the effectiveness and stealthy ability of the Predators. They are piloted remotely and vector in on their targets through GPS and precise satellite surveillance, basically. And they are stealthy. The Iranian airforce, if you can call it that, will never get a chance to respond. Assuming the strikes achieve their objectives, we have little to fear from Iranian retaliation because the highly centralized government will be paralyzed and in disarray without its top leaders -- which is the whole point for the venture.
As stated by several U.S. and Israeli officials and military analysts, bombing their nuclear facilities won't put an end to their nuclear quest. It will set them back, but won't put a stop to it and as long as the regime remains in power, they can easily reconstitute their nuclear program back together. To make matters worse, we aren't 100% sure if Iran's nuclear facilities are all accounted for. We have to assume that Iran has secret backup facilities hidden somewhere underground and that's the dilemma we are facing (see this eye-opening article:
Iran Uses Fear of Covert Nuclear Sites to Deter Attack).
Which is why we want to decapitate the leadership and allow the Iranian pro-democracy movement the opportunity to rise up and seize control of the government. It will be hard for Iran's regime to rebuild its nuke program if the regime itself is toppled.
They are also pro-nationalists and more often than not their nationalism and pro-Western stance aren't necessarily mutually inclusive and that's where it gets tricky.
The thing is that Iran used to be a democratic model until the CIA and M16 deposed their democratic government so British could own Iran’s oil. None of this would have happened if Mossadegh had not been overthrown.
Obviously a mistake, but we can't go back and rewrite history, unfortunately. But this doesn't change the fact that we're now facing a very dangerous, fanatical set of hombres who are working their asses off to build an operable nuclear weapon.
And then what? Even if we manage to dismantle their entire military, that still won't stop Iranians from retaliating through proxy cohorts in Iraq and beyond (and of course, pushing their nuclear program further underground). Remember, they would never intend to fight us in a conventional manner.
Again, if the regime is toppled and the pro-democracy movement takes control, these concerns will be minimal.
Since the primary goal of US policy toward Iran is regime change, putting boots on the ground will be necessary to achieve that goal.
It's unrealistic to think that airstrikes alone can effect regime change (it didn't work in Saddam-era Iraq).
Combat troops will be unnecessary, much like in Libya. We cut the head off the government by killing Ahmadinejad and the ruling clerics, target their primary command & control facilities and then support the rebels from the sky. It worked in Libya, it will work in Iran. Even more so, because Iran WANTS democracy and dissidents have been laying the groundwork for this for years.