My guess is some owner will give it a go. If KXSN a few years ago went to #1 originally as a Soft AC station, before it switched to Mainstream AC, and later Classic Hits, someone else will give it a shot too.
Could it be programmed on 105.7 XHPRS, whenever it comes back? Or will it return as a Classic Hits station? Few markets have more than one Classic Hits outlet. Maybe 105.7 will just be a non-commercial, automated juke box because nobody wants to pay what XHPRS's owner wants in return for the lease.
I'd say if Soft AC comes to San Diego, it may just be on 105.7. However, Northern San Diego County, which likely has older residents interested in Soft AC, might have trouble pulling in 105.7 from a tower in Mexico.