Why does Sion passive get weaker as he levels?
Mathematically, a Sion's passive gets worse as time goes on, because of the way health decrease scales quickly with levels. A Sion with 6 Warmogs at level 18 will last ~10 seconds in his passive, whereas a Sion with 6 Warmogs at level 1 will last ~25 seconds (without taking damage or auto attacking anything). (It also lasts longer with no items at all)
This means the level 18 Sion's passive is almost objectively worse, since the thing that damages you most is your own passive decay, and the majority of your damage in passive comes from the 10% max HP damage.
I don't understand why he is designed this way, since the infinite scaling of HP on his W implies Sion was supposed to scale with game length, like Veigar. Anyone have a good theory on why this is?