If https://en.wikipedia.org/wiki/Rape_in_Sweden#Legislation is to be believed, six years imprisonment is the maximum penalty for rape. At this point they could credit the majority of his sentence for time served.
Why should that count towards any prison term?
BTW, I don't think Assange would really face prison in Sweden. My best guess of what would have happened had he been extradited to Sweden:
An interview, being held a few weeks, then released to a country of his choosing.
He is inside the embassy because he likes the martyrdom that his followers clearly recognize. Not because there is any factual reason for it.
Seems like am aweful lot of money for such a small crime that you are making it out to be.
But perhaps there is other things that they are trying to deter people from doing.
Like, maybe they want to deter people from pissing off powerful governments by leaking and embarrassing world leaders?
Mind you, multiple American leaders have called for him to be arrested.
Maybe the giant elephant in the room has more than a little to do with why the government is spending so much money and effort trying to arrest him.
Sweden refuses to interview him there even though in the 4 years that he's been holed up in the embassy Swedish authorities have interviewed 44 other people in the UK. They also refused to promise to not extradite him to the USA where he would most likely be convicted of espionage and possibly executed.
If you were in his shoes do you think you would step outside given the circumstances? The UK has also stated that even if the Swedish investigation was closed they would still arrest Assange the minute he steps outside. Why wouldn't the UK extradite Assange to the USA if given the opportunity?