Cutting into a submarine pressure hull can affect its depth and remaining structural-life margins. An alteration can be technically possible without being a sensible investment if it shortens the safe or economical life of the boat. The same basic question applies to surface ships when new weapons, sensors, or launch systems demand space, depth, power, and major structural work. An upgrade is worthwhile only while it keeps the ship operationally viable for long enough to repay the money and dockyard effort. A navy must judge whether the resulting ship will deliver enough useful service, rather than merely whether an alteration can be made.
Notable Point · 22:22-24:26
Refitability has physical limits
A replacement decision turns on whether new systems can be fitted without exhausting the hull's useful life or consuming disproportionate dockyard effort.