Because it's HIS story.
I'm not sure if you get my point over here but I prefer to read the original author, regardless of whether his writing is not on par with his partner's. It is his own book and there's a certain "lack of matching" when it comes to reading the same story by a different author. Hope it's clear enough.
Read my point above. It's a bit complicated but I would definitely prefer his own writing. If her writing is good, I might try it out so my bad over there for not expressing my point correctly.
You seem to have missed what two of us pointed out: the series was very likely a cooperative effort. Some of his journalist colleagues have said that the writing of the books is better than his writing, which is an indication that they were probably co-written to some (unknown) extent.
I'm not saying you 'should' read any other books that might come out. That would be silly.
I'm saying that the original reasons you gave for not wanting to do so don't make sense. If his partner did, let's say for the sake of discussion, half the plotting and one-quarter of the writing, and three-quarters of the editing, then you HAVE read her writing and you HAVE read her version of the story, too. (To clarify, I'm not saying she did....I'm being hypothetical. We really don't know how much of her work is in the books.)
What if you found out the he did the plotting and she did all the character development? What if you found out that he wrote in synopsis fashion and she turned it into narrative prose? (Again, not saying this is the case. Just being hypothetical.)
If you don't want to read any more (hypothetical) Millenium books, that's grand. You have every right to choose what you do and don't want to read. But in this particular case, suggesting that it's for some reason of 'purity of Larsson's vision' is kind of silly: it's pretty well established that the books were to some degree collaborative.