If I needed to deal with tanks at level 16, I would've bought a void staff or rageblade before then.

Honestly I see people complaining that giving Kayle free true damage at 16 is OP, and I also see people saying it's a great buff. But I personally think it's going to hurt her that her power budget is wasted like this. Innate true damage and resistance pen are strongest in the mid game. Early game nobody really has resistances, and by the time you've hit late game everyone with some sense has already bought . If Kayle is having trouble dealing with tanks she cannot afford to wait until level 16; she has to buy a void staff or rageblade sooner. The problem here is that resistance pen does not synergize with true damage at all because you cannot penetrate resistances that don't exist. My ultimate point here is that true damage at 16 is a lot weaker than people think. It will primarily make a difference in lower elos where Kayle players wont be smart enough to buy %pen early and games are guaranteed to drag out to level 16.