A station on a frequency such as 103.9 would presumably do well to fill a niche and not go mainstream with a format such as Adult Hits, especially since there is already an Adult Hits station in the market on a 97kw signal. For instance, 101.1, 98.3, 104.3 are all serving narrow niches. Since there are so many FM signals in the market, there are just a few seemingly viable niche format holes in the Phoenix market in genres including AAA, Alt. Rock, Dance CHR (although 97.5 has been honing in on this format recently).
For instance, the alt. rock format is doing well in markets throughout the country. If it returned to 103.9 it wouldn't be the first time a format left and returned after finding out the grass wasn't as green as was originally speculated.
As has been posted here by others ad nauseum, the way 103.9 was executing the alt. format for the last few years of its existence was not at all in line with the alt. stations in other markets; instead it was a very gold-based presentation with literally the same exact tracks burnt into the ground day after day year after year. No station can sustain this in any format, not even classic rock.