It's core on every AD assassin
Intended
caster
Need you to define this. I can only think of GP and Jayce.
half the ADC roster.
ADCs abuse everything and get them nerfed. I'm done with them. Duskblade is core on 4 adcs (Jhin, Varus, Quinn, MF), and of those only Quinn gets Duskblade before youmuus (since she basically plays like an assassin). If duskblade were so busted, it would make sense for them to get it first.
Jhin has always gotten lethality, even before duskblade changes because it worked better on him than crit. Whenever Jhin has to build crit, it's because lethality ADCs are doing really poorly, and Jhin tends to end up doing poorly in those metas as a result.
Varus's poke style is just more fitting with lethality, not necessarily because of duskblade being OP.
MF has been using lethality+cleaver for a long time.
It's simply the best out of a long line of broken and overtuned AD items. There is nowhere even near to close an AP equivalent to Duskblade, which offers an AD caster literally everything they could want and more with practically no downside.
Apples to oranges, it doesn't really work. There's no AD equivalent of Rabadon's or Morello's for instance, because of how spread out the classes are for AD.
Mages need mana, CDR, AP, etc. They also need some sort of armor vs champs like Zed and Talon and Kha'Zix. if they go towards Morello, they lose all protection. If they go towards Zhonya's, they have no mana sustain and noticeably less damage.
Mages are immobile carries. Assassins are supposed to shut down carries, and do especially well vs immobile ones. While there may be a problem in how hard the assassins are stomping mages, it should come as no surprise that mages generally should be doing poorly against assassins.
Assassins are also pretty locked into their options. Take hexdrinker and shutdown their roam and ganks into non-Ap lanes. Taking away duskblade gets rid of a SIGNIFICANT amount of their burst dmg. If you have to nerf duskblade's burst dmg, assassins need some other way to increase dmg. Missing youmuus early on an assassin makes you lose out on an irreplacabl eamount of flexibility