When a proposal in the commitment stage reaches 45%, the SE team gets together for a review to determine if it would translate to a healthy site. I've seen a handful of proposals shut down at this stage; they don't just allow every proposal to make it through.
It was brought up in the early stages of this proposal that this site is a duplicate of Stack Overflow, Webmasters, Graphic Design, and UX. Here's the response from the SE team once that stage was reached: The future of the Web Design proposal. The overlap is specifically stated as a concern for the site:
Our main concern is that it seems like the vast majority of questions for this site could already have a home elsewhere on the Stack Exchange network, something that has already been brought up on Area 51. Questions about HTML, CSS and the like are already welcome on Stack Overflow. Questions about designing for function (rather than form) fit on User Experience. For this site to work out, the answers for a given question will have to be at least as good as what the appropriate existing site would have produced.
For the moment, the proposal is not going to be merged with any site. They're going to give it a trial run in private beta to see how it works out, then potentially move to public beta. I don't think the questions asked in the site's definition stage are all that stellar. Very few of them stand out as good questions that would be a good fit for Graphic Design, so it's difficult to determine exactly what the site's scope will be from that.
If the site gets shut down at any point, we can offer our site as a refuge for any decent on topic questions to be migrated here instead of erased from existence (this happened with one of the Arduino attempts). That's the only sort of "merge" I can foresee happening at this point. But first, they'll get their fair shot at it.