I do believe it's time to once again flip the armor pen formula around

Flat -----> %

instead of the current % -----> flat

This would delineate lethality builds from penetration builds completely.

As it stands, an ADC can build a ton of lethality and spike early, and when tanks become too much of an issue, tack on an LW or cleaver and still stay incredibly dangerous to tanks later game (while still dealing near true damage to squishies with all their lethality).

If you want to spike early, then you must do so with the mindset that if the game prolongs, you will get punished.