Because all of that has no third party actors. Here there are walking, talking victims accusing him of difficult-to-(dis)prove crimes. Assange has been shut up good since these allegations came out. Seems pretty effective to me.
If we're going to engage with the conspiracy theory angle (which I do think has some merit) then it's not about one single, definitive plan of action. It's actually about achieving a goal: stopping Assange from exposing secrets that various powerful people don't want exposed.
If you're willing to accept that the powerful do have secrets they don't want exposed then you should also be willing to accept that they probably don't care exactly HOW he's neutralized, just that he is. And judging by that criteria having him effectively locked up in the embassy is pretty good.
Utterly inept, you mean?
If they wanted to lock up Assange so badly, why not just stick some child porn on his laptop?
Or send a bomb threat coming from his email address?
Or punch someone in the face and claim Assange did it?
Or get him rip-roaring drunk, deny him use of a lavatory, and arrest him when the inevitable happens on the walk back to his hotel?
Or 5,001 variations on this theme.
In other words: I believe it's neither competent enough, nor incompetent enough to be staged.