Does the arch have a function while it is being built in your analogy ? Or does it only acquire a function when it is finished ? If so, then it still requires the foresight of intelligence to aim towards that final functioning state even though in the meantime it serves no purpose.
Behe certainly thinks the idea of multiple simulteneous mutations creating IC systems to be the biggest argument against his, since it is the main point of his last book to investigate this possibility.
Has it not occurred to you that perhaps he is fighting a strawman, and is "investigating" this possibility not because it's the biggest argument against him, but the weakest?
The obvious answer to Behe, which he has not to my knowledge tackled, is to point out that to show that something is irreducibly complex is to show that it couldn't have been produced by the last part of it popping into existence out of nothing.
But this is not what evolutionists claim to have happened in the first place. As Behe admitted: "There is an asymmetry between my current definition of irreducible complexity and the task facing natural selection".
Now we can see in the fossil record the evolution of something that is indubitably irreducibly complex --- the bones of the mammalian middle ear. This process did not involve the malleus, incus, or stapes poofing into existence, but a gradual modification of their forms and their relationships to each other and to other bones.
Now gradual modification is, you must admit, more evolutionary than poofing, or indeed the occurrence of simultaneous well-coordinated mutations, so maybe Behe could discuss that instead.
essentially, they propose a mechanism where mutations can neutraly accumulate in a gene, and then be 'revealed' all at once for selection, hopefully giving a worthwhile result that will be selected for.
Where do they say that this has anything to do with simultaneous mutations independently conspiring to form a biological system; or that it has anything to do with irreducible complexity; or that they've even heard of Behe's ideas?