Apple's whole business model is predicated on Asian slave labor.
How many American workers are going to want to work 15 hour days for $350/ month?
iPhone manufacturing will never come to the USA. It's too late.
Considering the amount of manufacturing happening in the US, it isn't impossible. It is just infeasible to assume those jobs are going to be like the assembly lines we see in China stocked with laborers. The manufacturing happening in the US is robotic. The manufacturing jobs are ones where people manage and operate the machines. So the labor cost of the individual is less important on a per unit cost, since you need fewer of them to manage a high volume of output. That or you are dealing with skilled workers that are working a craft rather than an instruction list, which is harder to value and outsource. Furniture made here for example.
iPhone manufacturing can come to the US, but assembly of electronics like this is notoriously difficult to automate (the circuit boards are already automated I believe). Which is why it stopped happening here. The main cost was labor at the assembly stage. If it did return, it's because the assembly part was automatable. Much like how the Mac Pro is done in the US mostly because assembly is far simpler, and probably automated.
But now that we do have this entire manufacturing pipeline in Asia, there are now even more costs of moving manufacturing out of the region. Since you no longer just have to worry about the cost of labor and the assembly stages, but the logistics side of things where you can no longer just ship cheaply from other parts of the region to get your supplies for assembly.
One of the biggest issues I see with manufacturing as an industry is that going forward, the biggest gains you will get in productivity isn't going to be by hiring more people. It is either more automation overseen by humans, or new technologies in automation like 3D printing enabling new forms of automation, in possibly smaller footprints. US manufacturing productivity is up, but jobs are down from the peak of around 20 million in the late 70s.
I personally wonder if we would be better off actually investing in these automation technologies to create local manufacturing jobs that way. It won't create as many jobs as moving existing manufacturing lines, but those are doomed IMO. Instead, it will be building towards a world leading manufacturing center. One that can compete in the coming decades.