Warp Lightning costs the same, is learned earlier, and will do far more damage on average
"Far more" ? Did you count it? Take an elite halberdier unit (not an unlikely target in early midgame), Contagion wipes it out, but they have a solid chance to just survive Warp Lightning. And that is without any positive enchantments! Elite slingers are another chapter in my book. Hammerhands, lizardmen units... and any unit with Lionheart... bah. Both spells are quite comparable, they excell against different targets. Maybe Warp Lightning edges Contagion because it can hit the big monsters. But Contagion is quite equal to Doom bolt, just cheaper. While you try to hold to principles elsewhere, why is then the most power/cost effective direct damage spell a non-chaotic one? It is really original as a spell, but the game logic is gone. It should be more expensive than Chaos, just like the Ice bolt is a more expensive (and later) variant of Fire bolt.
(And you have just chosen the most unbalanced chaos spell, others are worse, then you throw in Cracks Call - 'this lake looks shallow compared to Baikal' type of argumentation. Petrify is useless because unreliable = cost/time ineffective, and that shows how Contagion IS powerful: it does automatical damage, it is perfectly reliable when you need it.)
If you want to generally help death magic, you should IMO lower all units/creatures resistance by 1 or even by 2. That would turn many useless spells in the game into real alternatives.