They'll spend at least $5 billion on the project. That is enough money to completely remanufacture the entire arsenal several times using existing designs that are good enough.
They have been working on laser fusion since the 1970s, when they could test bomb designs by just setting them off. Indeed some of the early inertial confinement research was conducted using nukes as the radiation source.
Yes, it is true that the NIF data can be used to design and maintain bombs, but it is not required. We could provide a deterrent force without it. Therefore the real reasons are something else, my guess a combination of laser research, commercial and rocketry fusion research, and a large serving of national prestige.
