There is no particular rune causing this rampant damage creep

It's actually the bonus adaptive 15 AD or 25 AP gained from most of the secondary rune paths, in conjunction with the removal of the old armor and MR runes.

Which is why imo, those new secondary stat bonus's should be nerfed and or there should be a global buff to all champions base resistances, especially Armor..

The games broken af rn.