I fear that the decentralization argument has been misused in this way so many times that invoking it makes people assume it's an intellectually dishonest, roundabout argument that is selectively promoting states' rights in an attempt to accomplish some other, much more nefarious goal. This is really unfortunate, as I am a big believer in minimizing the role of the federal government wherever it's feasible and practical to do so.
Since you're the first to mention decentralization, I think I can be excused for not accounting for whatever it is you mean by it.
Isn't the power of government based on how many it "governs" and how many ("governed", "voters", "taxpayers") it controls?
I'm not sure how to answer your second question. Yes, but so what?