1) "They would be willing to create a global bloodbath to achieve their aims. They might not be willing to provoke a bloodbath that would result in the eradication of Islam with the US left standing at the end."
2) "I'm assuming at least some rationality in the sense of them wanting to achieve their objective, rather than victory-less suicide"
1)
If the scenario of the currency change comes to pass, there will be parts, numerous smallish enclaves, of America that survive - perhaps in my estimate one third of the population might get by but sink to virtual hunter gatherer levels. Internal territorial squabbles over food supplies, raids against each other would bring us back to the time of the pilgrims and the Wild Wild West.
We would be too busy killing each other over food or possessions, somewhat per the gang warfare among Crips and Bloods or mindless, senseless intergang rivalry we read about today. And barely fathom or feel.
The USA would not survive in the terms you envisage, nor would the Islamic countries spread around the world perish the way you portray or infer.
Most of these are at third world level and have a much greater agricultural rather than agro-Industrial profile, certainly not industrial. Like Ahmadi-Nejad and his tea, cheese and daily stew, they will get by with their chickens and ducks. Much as they do now. (Just to encapsulate).
They are far more likely to survive in large numbers than the USA and European populations. Even if their countries sink into chaos and robber barons and tribal chieftains exact tolls and rule certain areas like times of old.
2)
You mis-state their objectives, which are not rational - nor were in the past even to revolution founder Ayatollah Khomeini,who banned the Hojatieh sect that now rules Iran through Ahmadi-Nejad and his mentor Ayatollah Mesbah Yazdi.
Their sole objective is the return of their Messiah the 12th Imam. To achieve this they must destroy everything they can - THEMSELVES included - to create enough misery and suffering to get the attention of the 12th Imam and make it worthwile (in their fanatical minds) for him to bother with the world and return to redeem it.
NOTE: His return saves Islam and spreads it across the planet, so even if the USA were to survive, as you suggest, he would take care of that when he descends and turn everyone into Moslems. So why would that not be a TOTAL victory in their "unusual" minds?
You describe an objective that has logic and common sense as the basis. They have neither in their mix.
Remember, martyrdom is an honor and a sure path to eternal salvation in paradise. So why worry about being destroyed? Or in context care about destroying their fellow countrymen for the sake of their salvation? Voluntary or involuntary does not enter their minds. They are on a mission for Allah.
What makes you pause to even think they care? Because Ahmadi-Nejad does not rant and rave while foaming at the mouth on TV? He is not THAT kind of insane. He is the "cold-blooded", determined zealot, type of insane.
This is all so far removed from our culture, mind sets and beliefs that we fail to have a slightest concept of what goes on inside their heads. You have to have met them and lived among them. Or among similar ones. Usually small sects in distant parts of the world.
The closest you and most of us can come is to consider cults and our surface knowledge of them. But even then we do so at a distance, as spectators, never able to fathom their deepest thoughts and motivations.
You cannot do so using your Western mind - unless you can perform a wrenching shift from normal human patterns to those of an unfamiliar type of zealot you have never encountered and whom, until August 2005, you had no chance to observe, nor ever even heard about.
Anyone reading this has to shift mental gears when trying to assess what we face. For most of us it is beyond the range of our radar.