Let's assume hypothetically that holding DRK is riskless. In that case you have a riskless investment that has a return on equity of about 10%. Since that is obviously absurd, something must be wrong with this analysis.
It's often hard to argue with people about ponzi schemes because it seems like it should work and proving that it won't is difficult because we don't know the exact mechanism of failure in advance.
How exactly is Darkcoin a Ponzi scheme? The payment comes from block reward, not from new participants.