To be fair to the Swedish government, they can't offer a guarantee like this. Assange and his lawyers are taking advantage of this fact.
According to Swedish law, any application that comes in in the future has to be dealt with in court.
The government can't give guarantees that it will ignore the law in some hypothetical future situation. After all, the US hasn't asked for his extradition (yet).
Hm, perhaps, but what about other options? What about interviewing him in embassy in person or via Skype? Is there a reason that would be impossible for Swedish government?
No, I absolutely agree that would be a sensible and possible option, and I've read that the Swedish police have done it before in other recent cases.
I've not heard any compelling explanations why this hasn't been attempted, though IMO, Assange would be a fool to agree to it -- guilty or innocent.
On the other hand, there's little doubt that the attempted extradition procedure has correctly followed current UK law: it went all the way to the Supreme Court, and he was represented by absolutely top-class lawyers. So Sweden has the right to do what they're doing.
People have been extradited from the UK on 'European Arrest Warrants' for crimes such as drink-driving and theft of a piglet, with no prima facie evidence required.
They can officially recognize him as subject of political persecution and therefore would be unable to extradite him to the US under the principle of non-refoulement.
According to Swedish law, any application that comes in in the future has to be dealt with in court.
The government can't give guarantees that it will ignore the law in some hypothetical future situation. After all, the US hasn't asked for his extradition (yet).