Touche. But remember the number of assassins sent after Athyn Sareithi? And Mavon Drenim took several attempts to eliminate.
It may have been that the actor was safe 'for now' as the Tong would have had to reassess the situation, and find another agent capable or suitable to do the job.
True, but Athyn has some powerful enemies if they'd send 3 agents after him. It sounds like the agent was sent, failed, another was purchased, failed, and then a third.
Let's say a writ is put on your head by a rival businessman over conflicting trade routes. You survive the attempt by killing the assassin, and your rival doesn't have the money to send another assassin. Is everything over and done? After the attack, could you then go to the Tong, join, and sponsor a writ on the rival and carry it out yourself? Assuming the writ is purchased correctly.
Edit: Actually, if I were targeted by the Tong, could I then also sponsor a writ against the guy trying to kill me?