I sent think immigration will be so easy.
If your town has lost loads of npc pops that's not a good draw considering the npc are meant use maslows hierarchy of needs which has safety only just above food and shelter.
Plus travelling through the dangerous countryside, made even more dangerous by having an npc targeting player on the loose, could well mean that any npc that sets off may never arrive.
Add to this the time factor that your settlement could be missing vital services and what that could do to its viability, this making it even less appealing and you have plenty of reasons to assume immigration may well not be the solution.
Also who is to say that this is the only town suffering in this way?
Making npcs harder to permadeath seems like the solution to me but we will see what comes out of testing.