The real problem with Lethality.
Is not that the concept of the stat or even the amount that is granted by the items. Clearly the items are not all that efficient cost-wise, but they are extremely slot efficient. But even that isn't really the problem.
No, the bottom line is they just give way too much AD.
Consider for a moment what having 60 or 65 AD on an item means in a vacuum. We're talking raising AD on the order of 5 less than an infinity edge.
And you know what? Magic pen in all technicality is more powerful as a stat than lethality but it's not complained about because there's much less raw scaling involved.
Just think if we had 3 AP items that gave 15 flat magic pen each and all of them also gave 100 flat AP. That would also immediately be the dominant strategy for magic damage dealers like lethality is right now for AD characters.
What can we do about this?
First, cut all of the items down to 50 AD. I feel like that is an absolutely necessary change. Their price can come down to compensate, but it has to be less powerful late game than it is.
Second Make lethality reverse scale with bonus armor. Forget the scaling with level crap, that was a kinda ridiculous idea. Say someone has 70 lethality, and I've bought 100 bonus armor. Maybe we should make it so it scales like damage against armor. Like maybe that reduces the effectiveness of lethality by 50% so they now penetrate only 35. Duskblade can still have its full true damage, we're only talking about the physical damage amplification here.
I personally believe those are the 2 changes that need to happen to make it a healthier stat.