This isn't really between the US and Assange; it's between the US and UK. If the US doesn't honor the rules for extradition then the UK may decide not to extradite people in the future.
Most notably, the UK-US extradition treaty, which has exemptions for political offenses (e.g. espionage), has been found not to apply.
This article is decent https://theconversation.com/julian-assange-how-british-extra..., but from the middle of the trial. Craig Murray's blog is also a good source of info.