DMA57361's answer is the closest to describing the situation. (The other answers are right on what is on-topic or not, though. I really suggest you read our FAQ before posting such "let's change everything it doesn't work" topics.)
There are two important sides to keep in mind:
- The community: the group of people coming regularly, who are running the site, and providing a good half of answers.
They need a common topic, a common "reason" to stick on the site. For Super User, it's the fact to have a good place to ask computer questions, instead of losing hours searching on forums. This is also the reason which caught me, a year ago, and motivated me to help the site, by answering, and helping with moderation. This site is currently the best place, in my opinion, to ask about your computer related issues.
About the new sites, they are indeed playing this community aspect, on a closer way than Super User. There, it's really the feeling of belonging to a place where people use the same thing as you (Ubuntu, Apple, etc.). I guess your point is that since it will gather people more closely, questions related have nothing to do on Super User anymore. And this is where the second side is important to understand the whole picture:
- The people who just want an answer to their question: the persons who are searching for a particular matter on a search engine, and find a Stack Exchange site. Or they will find an existing question, and the answers with it, or they will find the site, as a place to ask their specific question.
To these persons, it doesn't matter where they find their answer. If they search about a Ubuntu problem, they couldn't care less where they found the solution. They just want a solution. And as long as they find it on the Stack Exchange network, it means we are actually "helping to make the Internet a better place"™.
This is why it doesn't matter if the sites overlap. Just because you have a site dedicated to Ubuntu, or Apple, doesn't mean you can't ask about them on Super User anymore. Super User welcomes all software and hardware questions (as long as they relate to your personal computer), that's the point of the site, that's its reason to be.
So, to answer your question: no, Super User is not dying, and is not even close to dying. It has a purpose, it has a community, and it attracts hundreds of new questions and answers every day.
