How to stop lethality+last whisper

Make Last Whisper (and its upgrades) proc armor pen only on critical strikes.

And maybe even add a bit of crit chance on it to make it a decent buy on its own, although I guess that's debatable. Adding 20% crit chance to Last Whisper (or its final forms) would cap normal ADC builds at 100% (20% from IE/ER, 30% from zeal upgrade, and 30% from second zeal item, and 20% from last whisper), too.

This would help prevent champions from building both lethality and % armor pen, although black cleaver still remains available.