This itself is viewed as a threat to our democracy by those who either think it should not be a republic or are ignorant of the fact it was intended to be. For reference, see debates about a non proportional senate and the power of minority population electoral power.
Nobody sane has ever cared about "states' rights" consistently - it's a silly concerpt. It was invented as part of the Lost Cause mythology to justify secession. (Fugitive slave laws, which are pretty antithetical to 'states rights," were very much something the antebellum South pushed for.)
Sure, ANY Republic does not rely on States, as evidenced by Singapore. But OURS (if you are also a USA citizen) does.
That said, the idea that "states rights" originated after the civil war is absurd on face vale. Powers and autonomy left to the states are evident in our founding documentation, and obviously present in the lead up to the civil war as well.
The idea that a centralized government leaves no powers to the state and local level is a silly concept and counterfactual. It is obvious in the interstate commerce clause and many other parts of the constitution.