I think it's safe to say that very few independent teams are safe from the ambitions of NHL teams, regardless of attendance. That said, the teams with the worst attendance are most likely to be targeted. If you were losing money year after year and an NHL team waved a bunch of it in your face, don't you think you'd at least consider selling?
Agreed. I do believe that those 4 AHL franchises are the most likely to be targeted by an NHL team in search of a closer affiliate.
I admit that Manchester's case was different. Despite their relative success at the box office, the fact that they were already owned by an NHL team (and a Pacific one at that) made them vulnerable to relocation.
As for the safest independent teams, number one has to be Hershey. Lake Erie and San Antonio are next safest due their being owned by NBA teams. Other safe teams include those which are already successful draws and close to their current affiliate (Charlotte, Grand Rapids, Lehigh Valley, Providence). After this group comes the three successful draws which are not closest to their affiliate (Chicago, Milwaukee, and Syracuse). These 3 markets are not likely to be completely vacated by the AHL, but it's possible they could be bought out by closer NHL teams and/or end up swapping franchises like Adirondack did after the 2013-14 season.
AHL franchises already owned by NHL teams but vulnerable to relocation include those which are mediocre draws and/or far from their parent (Albany, Bridgeport, and Hartford for the 1st reason; St. John's for the 2nd reason; and Utica for both). I think both Manitoba and San Jose are also vulnerable because they share an arena with their owner, which is not ideal from a scheduling perspective at least. If the Pacific Northwest ends up being colonized, I could see Bakersfield and Stockton potentially being drawn northward. The rest (Iowa, Ontario, Rochester, San Diego, Texas, Toronto, and Wilkes-Barre/Scranton) have little reason to relocate unless they prove to be unsuccessful draws, and even then their proximity to their parents gives them insurance.