Why people say Duskblade shouldn't be changed because "AD assassins rely on it to assassinate"?

This is actually a reason why this item should be changed. If all AD assassins are balanced around this item, to the point that they can't assassinate you at all without it, but with it, they can murder you even if they are behind, it shows there is something wrong.

Not to mention, this item removed or reduced the counterplay and/or the skill floor of some champions. For example, no longer has to rely on isolation to kill people, when Duskblade can do this job, he is probably the best abuser of Duskblade in the game.

This isn't even to mention the fact that so many AD champions that aren't assassins to begin with are purchasing this item just because it is that broken - sometimes they purchase it as a standalone item (without other lethality items in the build). This is wrong. Before, at least you were forced to use Duskblade in a dedicated lethality build to make the most out of this item, so those champions would be forced to sacrifice something (such as DPS) if they wanted to make the most out of Duskblade.

On my opinion just reverting Duskblade to what it was before (base damage significantly reduced, but it also has a 200% lethality scaling, and does true damage) would be enough to put it into a healthy direction. Or they could also revert it to its first incarnation where it dealt damage based on the enemy's missing health, but it was delayed (essentially a mini Zed ult).

f AD assassins prove to be weak due to this change, just give them compensation buffs; like what you did when you removed Deathfire Grasp (if Deathfire Grasp was removed due to being an unhealthy item, why can't they change Duskblade? I am not even asking to remove it).

I've seen people saying that my argument is not valid because Duskblade is to AD assassins what is for mages, or what is for ADCs - well, I also think that is unhealthy, but it is because it is too expensive, doesn't offer anything besides AP, is an uninteresing stat stick item, yet mages rely on it because they have been balanced around it and their damage will fall off late game without this item. About the last part, that's why Deathcap shouldn't be just removed directly. Until Riot rebalances all AP ratios in the game, they can't remove Deathcap. I would like Riot to revise that one day, however. As for Infinity Edge, well, crit is an unhealthy stat and everyone here knows why. If I am not mistaken, Riot said they will one day revise crit and AD itemization.