If a fix is relatively low cost and improves the software in a way that makes it easier to modify in the future, it makes it easier to change the requirements. In aggregate these pay off.
If a missile passes the long hurdles and hoops built into modern Defence T&E procurement it will only ever be considered out of spec once it fails.
For a good portion of platforms they will go into service, be used for a decade or longer, and not once will the design be modified before going end of life and replaced.
If you wanted to progressively iterate or improve on these platforms, then yes continual updates and investing in the eradication of tech debt is well worth the cost.
If you're strapping explosives attached to a rocket engine to your vehicle and pointing it at someone, there is merit in knowing it will behave exactly the same way it has done the past 1000 times.
Neither ethos in modifying a system is necessarily wrong, but you do have to choose which you're going with, and what the merits and drawbacks of that are.
