Buyer beware. Well no....buyer will do ok. User beware, your transactions are not anonymous
Your premise is that it's easy to buy the DRKs. However this goes contrary to market operation, in that escalating demand makes the price to skyrocket disproportionately as the available supply diminishes.
Ie the first one million DRKs you can purchase for, say, $3 per coin making you a bill of 3m usd. The second million coins will cost way more, like $30 (30mn usd). The third million will cost 300$ per coin to gather from 50% to 75% of the coins and that's at 300mn usd for that million.
The last 25% will be in the billions in terms of price. And then you have to buy the daily supply of, say, 7000 coins x 1000 usd each = 7mn per day... And if you do that,
then you have to do that also to every coin having DarkSend implemented. So every coin that launches (or implements) DarkSend will be a quick target for NSA acquisition, so it's a guaranteed price success, so new clone coins would be guaranteed sell-offs to the NSA which will be, in a sense, turned to cryptoland's cashcow which we we will be gladly milking as we sell our DarkSend enabled coins and mine new ones etc. And the NSA will do that, while, as you say, there is cryptonote out there, or zerocoin tech etc etc.
From a game theory perspective, they'll have to find another vector. This would be too costly with too little reward, even for their budget - primarily because it's a never-ending game.