First of all, I was thrilled to hear that there was going to be a pure dark-type (or any dark-type, for that matter) legendary, and incredibly pissed off that it was going to be so hard to get, so that's a complete no on changing him to purely ghost. Second, why would you want a legendary pokemon with no weaknesses? You would probably never see a team without one again, especially with Dark Void and Bad Dreams.
Next, it's not rare to have a pokemon with a single type, but you can't call it unoriginal, either. A single type is a good idea for any legendary, unless it's unusual AND particularly useful, like the meeting of steel and dragon for Dialga. So, now that there're already TWO pokemon with that type combo (one of which is actually used fairly often), why would you want to do that to them? What's the next step, making a Normal and Flying legendary? And yes, I know, not all legendary pokemon have original type combos, but we don't need to keep that going.
Also, you can't lower its stats too much to make up for the type combo, because its stats already put it at a slight disadvantage in the Uber tier, as they are slightly lower than average for a legend (probably to make up in part for Dark Void).