The thing is, NPCs are never required to progress. Sure it can be great if that's something you want to focus on, but it's not required. Just going into a couple of cave systems nets you some NPCs pretty darn fast, so that's where I don't see a huge issue with the actual mechanics. The two potential problem spots, to me, are:
1. Is the seeding frequency for NPC rescue missions still too low (we adjusted this once already), or are people just looking right past them?
2. Are we not properly communicating that people ought to be going into the caves to find people more easily?