In part, this is because documentation has never been sufficiently exact to ensure replication. As a case in point, the United States attempted to remanufacture both the W52 and W68 warheads on the basis of simulations. However, when actually tested, both weapons had a measured yield well short of what test-experienced weapons designers predicted. This is a lesson that the administration, in supporting the CTBT, seems willing to forget. The CTBT also will prevent the United States from developing new weapons to counter new technological advances by adversaries.
Nuclear testing is essential to such modernization.
Without it, the nuclear triad will become obsolete. I fail to see the logic behind the argument that the United States has no need to modernize its deterrent if Russia, China, and others are similarly constrained. Historically, this simply has not been the case. Indeed, nuclear weapons modernization is generally driven either by new mission requirements, or by non-nuclear technological evolution in defensive systems. For instance, during the cold war, advances in air defense and anti-submarine warfare created needs for new weapons.