The US democracy is not threatened by violence against any single building or even most sets of people.
Had they burned the building to the ground and somehow caused the deaths of everyone inside, the US democracy would continue. They did not achieve anything remotely close to that.
There is no basis for calling some angry rioters smashing windows and smoking pot "a substantial threat to democracy". None.
Protests that turn destructive are bad, no doubt about it. But the hyperbole is painful.
https://www.huffpost.com/entry/black-panthers-california-196...
They came armed (nominal, as they went armed everywhere) to a peaceful prearranged meeting at the statehouse. The media turned it into a crazy narrative because black men carrying long arms were photographed inside the statehouse.