Against a really clever predator, perhaps with wings? Against accident and illness? Against a change in circumstance such that those overspecialized strategies no longer work sufficiently well or at all?

A colony could stockpile food for the interrim period where the backup could be trained up.

Trained up by whom? All that's left of the colony's institutional knowledge on the subject of provisioning is an owl pellet's worth of tiny bones and hair.

With immortality, institutional knowledge doesn't have to go away. Everybody can know everything, and the specialist that has all the experience dies, then a new one can just be selected, and the whole colony can pick up the slack until the new specialist is experienced enough to do it all himself.

Everybody becomes T-shaped.

So immortality implies perfect recall, such that skills not exercised cease to grow rusty and fade with time?