Probably nothing to do with statutes of limitations, as there are other charges, that is just and excuse.
The UK has been spending a small fortune providing 24 hr guard on the embassy to ensure that Assange does not sneak out. This is causing issues in the UK. So the UK will have put pressure on Sweden to resolve the impasse.
Assange would have been in and out of relatively comfortable Swedish jails by now if he was found guilty. So one must assume that he at least believes the rendition issue is real.
Also, I understand that Swedish sex trials are held in secret, with no jury, and 2 "lay" judges that are appointed by political parties. So it is likely that he would be found guilty of whatever he was charged with. That said, it would be embarrassing for Sweden if the case was too outrageous.
The fact that Assange was allowed to leave Sweden in the first place is clear evidence that the case against him is weak at best. Certainly not a case of two women screaming "Rape!" as Assange beats them up and rapes them.