Why leave the main project (without closing this project) and move on to developing a secondary one (the creation of which implies that this project is better, otherwise why create it at all)?
It wouldn't be a good idea to ever close either project as both still have massive room for improvement, especially as technology advances. I really don't know.
Mind you there is no explicit evidence that satoshi was actually involved in the development of Namecoin, or that he and Vincent Durham (thought to be a pseudonym), the first developer of Namecoin, are the same person.
Maybe the whole point is that Satoshi has nothing to do with this at all? 
Yeah there is no actual proof that he did. Its just as good as a conspiracy theory as any other I've heard, so that's why I like it. Its better than Craig Wright saying the thing was Metanet, which turned out to be a big nothingburger. How surprising.