SSGN missile capacity and the cost to the attack-submarine fleet

What is the purpose of an SSGN, and why not use attack submarines or surface ships instead?

An SSGN concentrates cruise-missile firepower, but loading that role onto enlarged attack boats can change their employment and increase pressure on attack-submarine construction.

An SSGN concentrates a large cruise-missile battery in a submarine, creating a different operational asset from a boat focused on fighting enemy submarines and surface ships. Clarke and Drachinifel question whether repeated extensions of the Virginia design are the best way to obtain that capacity. Their concerns include hull proportions, handling, and the extra construction time required while older Los Angeles-class boats are leaving service. A Virginia carrying a particularly valuable strike payload could also be withheld from combat that an ordinary attack boat would undertake. Incremental Virginia upgrades are defended as easier to obtain politically than approval for another submarine class, and as a way to obtain missiles sooner. The proposed alternatives are a cruise-missile version at the end of Columbia production and further conversions of Ohio boats with sufficient remaining service life. Both depend on funding and usable hull life.