Okay, let's look at a hypothetical - you say the investigation was dropped because they couldn't interview him. What is the practical difference to the investigators if they interview him and he says "I refuse to comment on anything"? There must be some way to move forward without cooperation from the accused - Assange isn't the first person to flee a country pending an investigation. Why didn't they do so?

Looking at Swedish law, they have a rough equivalent of Miranda - https://open.karnovgroup.se/processratt/fuk Section 12(google translate):

- Do not have to comment on the suspicion and not otherwise have to contribute to the investigation of their own debt