So yes, neither Darkcoin, Cloak, Monero, or any of the other so called anonymous coins will ever gain mainstream adoption. The future for cryptocurrency transactions is Bitcoin. The coin that overtakes Bitcoin will be a coin that is faster, smaller, so easy to use any idiot will be able to, etc... It won't be an anonymous coin.
Your line of thought is very limited in scope. Nobody expects DRK to replace bitcoin in terms of volume of transactions.
Let bitcoin have 90-95% of all transactions with a 10bn marketcap crypto market and with just 5-10% of transactions through anonymous coins, you'll have 500mn - 1bn marketcap for them. It's a niche market, covering a different type of need.
Yes, we get it. You don't want anonymity. Some want it. Some (=billions of people in oppressive countries) will NEED it to bypass their country regulators. That's what at stake.