To Everyone On the DFG vs Assassin Dagger Debate

First, before you try to compare the two, understand that while they both fulfill the purpose of increasing damage output, DFG was removed for a very specific reason.

In the case of Deathfire Grasp, the item itself was a balance nightmare. See, there is a fundamental difference between the role of an assassin and the role of a mage. Most mages are good at doing damage than can be bursty, but the majority of mages can't insta-gimp a squishy before they have time to retreat to their tower. Their burst is oriented towards teamfights and generally just causing mayhem with every ability for the opposing team.

Assassins, on the other hand, are not oriented to teamfights in the way mages are, they thrive instead on picking off lone targets out of position, excelling at being able to get in, get the kill, then get out. Assassins have extremely good burst single-target, but they have to risk themselves to get in and can be dealt with then and there by a prepared team.

Deathfire Grasp essentially turned more than a few mages into long-range assassins.

Deathfire Grasp promoted a style of mages that would simply seek out a target that wasn't a tank, use the active to take a small chunk of their health, then dump every cooldown on them without any thought, and more than likely the target was dead, and then the mage just left, having accomplished the role of an assassin despite not being fundamentally designed as such.

It's like giving marksmen an item with stats designed for them, with means of mitigating all incoming damage AND ignoring crowd control for a few seconds, which would promote marksman to start playing as fighters would, and make balancing certain champions a complete and utter nightmare (there is a good chance Urgot of all champions, with his own passive to reduce damage, would probably jump to the top of the Flavor of the Month club with an item like that).

Meanwhile, the only champions who benefit from the item Assassin Dagger are Assassins and Fighters who thrive on Armor Penetration, like Gangplank and Pantheon, because ADC's need to auto-attack and can't choose when to proc the passive.