Duskblade compared to damage dealing AP items

You know what all of these items have in common? They all have AP scaling. Althought its definitely strong on them, Assassins aren't the worst abusers of at the moment, fighters/bruisers are. They can buy a Duskblade and get the same damage from the passive as an Assassin. Hell, the meta could potentially turn into a few Assassins buying the item for guaranteed baseline damage followed up with going tank/bruiser for the rest of their build. Meanwhile the items above clearly get more damage in the hands of an actual carry, and tanks/bruisers will rarely, if ever, buy any of them except in cases like and .

Why, then, did the Duskblade change launch with high base damage and no AD ratio if the item was meant for champions that are supposed to focus on offensive stats? Hell, now that I think about it, none of the lethality items actually have much direct synergy with eachother aside from the raw lethality, which doesn't make sense considering Riot's expressed desire to give AD assassins their own itemization style separate from bruisers and ADCs to avoid collateral damage when balancing the 3 classes against one another.