Our society idolises the independent individual with consumer power and endless choices. Retirement looks like an ebbing tide withdrawing such freedom from reach.
All creatures so far created something beyond themselves; and you want to be the ebb of this great flood and would even rather go back to animals than overcome humans?
You would only really need to patrol the walls during low tide, but even then the zombies would struggle to scale up the hills and the walls that surround the island on all sides.
The morning surge is a blessing, obviously, but there is also something comforting in the ebb tide of afternoon, which a cup of tea can gently reverse.