Balancing present capability with future growth margin

What makes a warship last longer: maximum capability now or space for future improvement?

Both matter; a durable design balances a credible initial fit with enough margin for later upgrades.

A long-lived warship needs useful capability when it enters service and space for later improvements. Reserving that space means balancing immediate requirements against equipment that has yet to arrive. Getting the balance right is difficult, but making that allowance is worthwhile.