It lets you have a middle ground, with the decision of when breaking happens left up to library users.
Without non_exhaustive, all consumers always get your second scenario. With non_exhaustive, individual zoos get to pick their own policy of when/if animals should starve.
Each option has its place, it depends on context. Does the creator of the type want/need strictness from all their consumers, or can this call be left up to each consumer to make? The lint puts strictness back on the table as an opt-in for individual users.