You realize that submarine was only meant to last 20 years and instead they spent 3 years rebuilding it to serve a different role? It is also closer to 40 years than 50 and is decades younger than the rockets in question. I'd also hazard that the lifetime maintenance costs of that submarine far dwarf it's initial construction costs.

Part of the reason why the SLS took so long and cost so much is because they DID try to re-use all the old resources and technology rather than building from scratch.