He would, no doubt, be given a lengthy federal trial, likely found guilty and serve Federal prison time, while receiving moral and legal aid from many corners, particularly organizations concerned with civil rights, whistleblowing, and journalism. His conviction might even be eventually overturned.
Indefinite detention works (in that it has not been stopped) because those who are imprisoned without due process have few mainstream defenders - plenty of people object to the methods, but are ho-hum on the actual people involved. Assange's detention without due process would be a political nightmare for the administration at very least.