So do you think Qt represents a significant threat for MS or not? I'd say that it must represent a very significant threat if they are willing to spend "billions" in trying to derail it.
Why would it be the worst case scenario for KDE to fork Qt? I don't think you are making any sense. It would much worst if Qt is allowed to stagnate without any one picking it up.
As for MS, the worst case scenario is for Qt to become even stronger outside of Nokia's control. I personally think that is what is going to happen.