I think it would be fair to stop prosecuting him if he stopped being an active adversary of the US.
I’m a firm supporter of holding the US government accountable when they do bad things, however, Assange is most definitely not someone who is partial or somehow morally correct in his behavior.
During his years of activity, he has been a willing conduit for a number of foreign intelligence services and has actively put his thumb on the scales to harm US interests. His closeness and willingness to launder FSB, GRU and MSS data stolen from western governments make him a legitimate target for the US government.
He isn’t a hero like some people want to believe. Fuck that guy.