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> I can see the need for maintaining what we have already [...]

The officially-stated goal of these labs (pulsed power, fusion, and hydrodynamic test facilities [0]) is indeed for maintaining existing nuclear weapons, not to design new ones (and also for doing basic research during free time). This was called the Science Based Stockpile Stewardship program [1] - ensure that existing nuclear weapons would remain functional in the foreseeable future. (Interestingly, the lesser-known hydrodynamic test facilities such as the Dual-Axis Radiographic Hydrodynamic Test Facility are more useful for weapon designs than fusion facilities).

The idea is to test materials under extreme lab conditions to help computer modeling, so that it would still be possible to do minor design changes to replace obsolete or end-of-life parts (the FOGBANK incident [1] came to mind). Understanding long-term aging is also a stated goal.

[0] http://www.wslfweb.org/docs/agex.htm

[1] https://en.wikipedia.org/wiki/Stockpile_stewardship

[2] https://en.wikipedia.org/wiki/Fogbank