“A general in chief, should ask himself frequently in the day, what should I do if the enemy’s army appeared now in my front, or on my right, or my left? If he have any difficulty in answering these questions, he is ill posted, and should seek to remedy it.” – Napoleon Bonaparte
That quote does not (just) mean have a plan; it means have a plan for when things don’t go according to plan.
In a financial planning context, “I plan to work until 65, and with my current savings, savings rate, and spending level, my projections show I can retire comfortably” is (assuming the assumptions are reasonable) a solid plan. But what if the individual can’t work until 65? Then they are “ill posted” and should “remedy it” – perhaps with disability insurance, perhaps with additional education or training (to ensure they are not made superfluous by AI or other developments), perhaps by making sure their parents have adequate LTC plans (so the individual doesn’t have to quit or cut back on their work to care for them). You may be able to think of other potential threats to the “work until 65” plan.
I think financial (and other) planning is an extended exercise in seriously engaging with the question: What could go wrong?