"deal with" doesn't have to mean killed/destroyed/etc. His capture is a form of "dealing with" and doing so in a single patch. Sargeras isn't some parasitic being leeching into a planet oozing their essence to manifest beyond their confines so comparing to old god parasites isn't a good example.
When he does break loose again and they retcon his motivations (yet again) it will still be a further let down.