Damage creep and defensive items&runes have a thing going on, conqueror is in that mess as well.
As damage is increased the wish for stronger defensive options arise, but when defenses outperform damage the option becomes to once again increase damage to the point where the new defenses cant endure.
However this kinda bones everyone who doesnt have or use the defenses, this is where we were before conqueror, particularly in the toplane for many melee trying to lane against tanks who kept screwing with them by rushing resistances and smacking them around with aftershock.
So to avoid the scenario in which damage rises exponentially to counter defenses they added conqueror as a means to bypass defenses without actually adding a ton of bonus damage to everyone.
But that naturally makes the tanks feel bad as they can never hope to deal with any conqueror user cleanly, and that is where we are now.
As for lethality, its not actually a necessary stat, you could just delete it and buff the users and it would be a done deal, however and understandably, riot wants each class to have their own set of "core items" to pick from.
Now the bugger here is, lethality offers increased damage by means of negating armor and its especially effective against targets with little armor or health to start with, the items with lethality on them usually have flat AD, some cdr along with a passive&active ontop.
This naturally benefits AD assassins as they scale nicely of both the stats and the passive&active.
However there are other AD champs who also like burst or have abilities that scale with AD very nicely, like Jarvan or MF, both like when their abilities deal more damage and both want that power as soon as possible.......much like assassins actually.
Added bonus when their synergy with the commonly used items for their class isn't the best for one reason or another.
So the talk about one class abusing items meant for another is in my mind generally kinda nonsense, multiple classes have champs whose traits overlap with other classes, like Diana or mundo for a pair of examples, this means that those champs will often have good synergy with items meant for both classes or maybe only for the ones with those they are overlapping.
Now the big issue as i see it is that tanks&marksmen&supports still outscale everything so hard that they basically have to have a near disabled early game to compensate and others get to have a powerful early as to differentiate them.
However this naturally results in a case of those 3 who scales well never getting the chance to actually scale, and when they do everyone else feel like shit because they dont have much of an impact if they cant end in time, they just end up getting chain CC'ed and melted.
The ones being the most at odds are probably tanks and assassins, they simply cant be good at the same time as tanks just outscale and shut the assassins down permanently when good, however when assassins have the upper hand they just deny the tanks a chance of ever scaling up by stomping them early and ending the game before it gets anywhere.