It could make sense to just increase their spawns as time progresses and then decrease them over a longer period of time. Think a bell curve with a positive skew. They should still be rare (rare enough to make it not weird that you don’t really see many survivors), but in my opinion this would be a decent model for the spawning rate. Perhaps better gear could be found as time progresses, representing the progress of the survivors before they died?
The skew is there to show that the idiots die first and in numbers (heck, they could even be indistinguishable from regular zombies what with the current loot spawns), and then as time went by the survival of the fittest takes over.
It wouldn’t make sense to me for ALL of the survivors to be dead within the first five days, especially seeing as the evac shelter’s supplies apparently lasted you this long.
It’d be interesting to find journals, notes, etc. on the bodies. We could even tell a story with the regular items in the game. A boarded up house with a single dead (EDIT: undead?) body and a gun with no ammunition except one spent casing on the floor for instance.
If I’m unclear on anything just ask me to explain what I mean.