I don't think the argument should be over whether Iowa is a swing state (South Carolina clearly isn't but it gets no.3)
The argument is frankly what order of states best suits the parties ability to win 270 electoral votes & how it best stress tests those running.
Those two things are inseparable - and the purpose of the two first states is different from the purpose of the third and fourth.
The purpose of the two first states is to find a candidate that can connect with voters in "Middle America" and win in swing states. SC is a corrective to make sure that you don't get a nominee with little appeal to black voters (NV serves the same purpose wrt Hispanics), since that could depress turnout.
If you keep lily-white NH as one of the first two, the second one needs to be a swing state that's at least a bit more diverse and includes a large metro area. On paper WI, MI, NC and GA are the obvious choices, but MI/NC/GA are too big for retail politics (all with roughly twice the population of SC). AZ is a large state with a 7 mio.+ population, which is a also a bit too big, and you already have NV on the calendar. WI is only slightly bigger than SC and in the same region as IA, so that's the obvious replacement even if it's slightly too white, but if you keep NV/SC as correctives that's less of a problem. NH/WI followed by NV/SC is the least radical change that'll still "do the job".