As in the bombers example, the previous generation is hardly quickly and completely outdated. They're still nukes. And that's exactly the reason to keep them in service.
Then, when a better tech becomes an option, it doesn't replace the previous weapon, it complements it.
So the options are: 1) retire old weapons, recycle them to build weapons which are expected to be better, keep the military potential about the same, reduce costs but still spend a lot and 2) keep what's in service and build a better weapon. Option 1 is hard to justify politicaly. Scraping something perfectly functional that costs billions sounds like a career ending decision. It also affects too many people. Option 2 is almost a guaranteed way to improve military potential, and a way to have a stellar career. Assuming that funding is available.
So yes, the decisions are not always the most economically efficient. That's a wrong objective function. Especially if we're talking about defense.
If the new weapon systems are about as good as the old ones then what’s the point of building them? Presumably there is a significant threshold for spending 100’s of billions of dollars on a significantly improved nuclear stockpile.
Justifying it to the American taxpayer is meaningless when it’s hidden from them. But, justifying it to Congress was just a question of pork spending which is really what the excessive nuclear stockpile was. It’s no accident that all 3 major branches of the military had Nukes, though currently that’s down to Air Force and Navy.
Then, when a better tech becomes an option, it doesn't replace the previous weapon, it complements it.
So the options are: 1) retire old weapons, recycle them to build weapons which are expected to be better, keep the military potential about the same, reduce costs but still spend a lot and 2) keep what's in service and build a better weapon. Option 1 is hard to justify politicaly. Scraping something perfectly functional that costs billions sounds like a career ending decision. It also affects too many people. Option 2 is almost a guaranteed way to improve military potential, and a way to have a stellar career. Assuming that funding is available.
So yes, the decisions are not always the most economically efficient. That's a wrong objective function. Especially if we're talking about defense.