You're making a couple of pretty strong assumptions with that statement. One, that Iran has enough of these things to make a difference, and two, that we are totally unable to defense them. On the first assumption, if Iran's test was acceptable, they no doubt will be making these things as fast as they can. But we, that is, you and I, have no way of knowing just how long it takes and what is required. If however, this device is at least partly based on the Russian design, which seems highly likely, then our military has a pretty good base of info from which to work.
Secondly, as to whether or not we can mount a successful defense against them, I would guess that whether yes or no, our military will be relatively quiet about it, but at the same time, speak - publicly, that is - in only reassuring tones. Since we have known about the principles since the Russian design became known, it would seem quite unlikely that we haven't studied how to defend against them. Whether our studies bore fruit or not, only the Navy and the DoD know for sure. If we lose a ship or two, we may find out.
Do they pose an incremental risk? Absolutely. But do they signal the end of the Fifth Fleet? Not hardly.