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I wonder how much of this is really up to KDE?
I believe I've seen distros (like pclos, maybe) which put some of their tools in System Settings, or at least links to them.

I think all that KDE can really do here is make sure that System Settings provides a way for distros to add their tools. It is mostly up to the distros to decide to add their stuff to System Settings.

Take the kdeneon distribution as an example (or kubuntu): here the settings ARE integrated, only one tool exists. So this IS already possible.