I think many of the units shouldn't obsolete. In the world of Erebus, in many cases a northern border city's greatest threat is goblin raiders coming from the depths of the tundra/snow you can't be bothered to colonize, and a few bronze/iron wielding warriors, or an archer or two, will totally suffice, whereas a larger hammer investment just to defend against occasional raiders isn't worth it.
I think there should never be a time where 1) I cannot build my generic fighters/archers and 2) a new limited-number unit, like Knights, overrides a useful unit like Horse Archers. Both of these are bad, and limit a player's strategic options rather than extending them.