Lethality is a LOT weaker than armor pen was though - at least for some champions.
MAYBE it's okay on something like Zed. The problem may be solved for him where if he falls behind he's still relevant with his standard build. For some reason it was okay for him to force every mage to build Zhonya's regardless of how well they played but it's not reasonable to expect him to not go an armor-pen build if he falls behind...
Anyways, the point I want to make is that armor pen was something that you COULD choose to invest in if you got an early lead. It was something you explicitly took because it offered early power before your opponent has resistances. It was something champions with an oppressive laning phase could use to start snowballing before they simply got outscaled.
Now, there's no such option. You can't invest in an early power spike with Lethality because by definition it does less at lower levels. It even scales at your opponent's level, so it's not like getting way ahead makes it any better. You're almost definitely better off rushing flat AD than you are rushing Lethality now in these circumstances.
It's definitely a net-nerf to champions who had armor pen as an optional build for when they were doing well, but could opt in to alternative builds if they weren't.
The top 6 win-rate champions now are all tanks. 5 of the 6 have weak laning phases (exception being Sion). I think that proves my point. If it doesn't - look how Jarvan 4 fell from 1st to 26th on just this one patch. His optimal runes were 9 armor pen marks which let him snowball an early lead when coupled with the armor shred from his Q and black cleaver. Now lethality denies that snowball combination, even if the mastery changes probably benefited him. Quinn fell from 2nd to 8th behind the same premise - she can't snowball (as well) with early armor pen off an early lead any more.