As it stands, DarkCoin can live forever, if the artificial 84million cap were removed. It is still rare, it takes a century to get to 100 million. Look at the United States Deficit! It's in the Trillions! And people think 100 million coins for the world population is too much?
I'm going to respectfully disagree. Darkcoin needs a cap, it doesn't have to be quick or sharp, or anytime soon, but there needs to be an expectation that it's a limited commodity like gold. Why? Because it's directly competing with other limited commodity cryptocurrencies.
If darkcoin is inflationary, there is no advantage to keeping your money in it long term. This is a problem, because trying to trade in and out of the currency would reduce the privacy that it affords. I could hold another limited commodity cryptocurrency instead and very likely watch its value gain with time.
Gradually reducing the rewards over time is an excellent solution. Miners will still make money off of transaction fees, the price will adjust upwards as inflation diminishes, and there will be real incentive to keeping your money in darkcoin long term. These are the characteristics you would want in an ideal currency.