They would need to make a lot of transactions to be successful. And once ring signature will raise it will make their work exponentially harder. Also lets say two agencies want to do this attack. KGB and CIA. While one of them is adding transactions it is just making other agency work harder.
I doubt is the large scale attack Anonymity outlined in his hit piece, since as you say, with modern ring sizes and more than one attacker it becomes impossible. They are almost certainly dusting wallets of interest.