Adding delayed damage to assassins was, and still is a good idea, you should be able to react before dying. Where the problem lies is that other classes can still delete you without delay. Right now, if you want to fulfill the fantasy of oneshotting someone, you might as well play a mage, or heck even an adc. Oh look, Lux landed a Q on you and has her skills up? Rip. Vayne as Infinity Edge, and static shiv? You better not get auto attacked 3 times! Caitlyn landed her net on you and autoed you? Rip half your health. Meanwhile Leblanc has the time to die 3 times before she can proc her mark.