You don't understand.
The idea is definitely cool and interesting on paper, however the lethality scalings would probably have to be quite impactful to overcome the "nerf" lethality would take when built on other champions.
If not, you would barely be making a difference and other champions would still be building lethality anyway.
So, with that in mind, the moment you add lethality scalings, you have to take power elsewhere from their kits - like base damage or their AD scaling, or whatever.
Whatever change they'd go for, it will mean that without said lethality items, these AD Assassin's would be in a much worse spot than they are now because they would be basically nerfing themselves if they don't buy lethality.
And yes, Diversity doesn't have to be forced. but I don't want to be forced to build the same 2 items either.
If I want to experiment a weird build, I'd rather fail because this weird build doesn't offer me as many Pro's as the regular one.
I don't want to fail because it's simply impossible to scale without <item>.
There's a difference between those 2 cases.
Do you understand what I mean now?