The way to stop the extradition of Assange to Sweden would be to have a more serious charge of espionage brought against him by the US.
If NATO did not exist, there would be no treaty to supersede UK law and you could be correct.
But the treaty does exist compelling extradition of spies.
Assange, if alive, will be extradited to the US where he will be a witness, and he will be cleared because he did nothing wrong.
The revival of the bogus Sweden rape charges is a bid by globalists to prevent his extradition to the US and so prevent his testimony. The treaty does not compel extradition for rape charges.
Assange’s dilemma is he cannot be guaranteed US immunity in exchange for testimony else the Democrats would claim interference.
If Assange’s health is as dire as reported, the matter of extradition will be moot.