Could a civilian shoot it down?
At 12 miles high?
A few rounds from a 30-06 or even a .45 1911 handgun at close enough range would start the deflation and it would come down. That civilian would have to have a private small aircraft or helo take him within range. Could, sure. But nobody did.
Concern may have been that it was laced with a virus or some other poison.
The USAF should have been tasked with its removal as soon as it passed into US airspace. Under Trump, it would have been so.