They're also the nation's timekeeepers and weightkeepers and lengthkeepers. (https://www.nist.gov/pml/owm). IIRC if you think your 1kg weight is sus, you can ask NIST for the reference 1kg weight and weigh your 1kg against NIST's 1kg, similarly for lengths. It's NIST's job to ensure their 1kg stays as 1kg and does not lose weight due to any sort of erosion.