Why a modern Japanese destroyer needs missile capacity in reserve

How would the design advice change if the question concerned a modern Japanese destroyer?

Start with its likely future threats and allow a generous VLS margin instead of limiting missile capacity to the immediate requirement.

A modern Japanese destroyer successor would need a fresh assessment of the threats it is likely to face. The relatively predictable requirements used to discuss a World War II destroyer do not settle a present-day design. Vertical-launch capacity is one of the main concerns raised. The design preference here is to carry more missile cells than the immediate requirement appears to demand, leaving a margin against future needs.