@TOMATO_QT
You pretty much have the answer to your question there.
SO does not help one bit in keeping a community together. Also fragmenting the services does not help in that (Q&A in SO, other things here), and people would probably still cross post to both (or shout for a Q&A area here).
I don't see having multiple Q&A forums an issue (there's Qt centre, Qtforum, Qt apps, Qt developpez, Qtfr, and other local sites too). The thing we try to do here, is to be a place where the community can come together, talk and ask questions among other things.