I don't deny that level 5 magic is good. Everyone agrees that level 5 is great. I was talking about everything else. Everyone wants to reach level 5, but the path to the stars is thorny: you don't know for sure if, for example, Order will have the "Mirror of Pain" in the city, and you don't know for sure if you'll have time to upgrade the Grand Mage by the final. Therefore, the key level when deciding whether or not to upgrade the Grand Mage is level 4 magic. Levels 2 and 3 are easily obtained even with a tactical approach.
Well, why is it not the same? Take any other school, and you'll see the same thing. Nature is mostly just summons, similarly excluding the "Hive" of wasps, "Stone Skin," and other spells (including those that copy Chaos magic)... I mentioned the specifics for other schools above.
If you think that getting "Disintegrate" immediately makes "Lightning," "Fireball," and the like irrelevant, then in online play, you will inevitably encounter the problem of a widespread mana shortage. Just look at the damage-to-cost ratio of the spells, and you'll see that increasing the level of a damage spell doesn't always improve this ratio. In battle, it's more profitable to use a level 1 spell; you save mana.
Saving mana makes sense only in battles with neutral creatures; why save it in the final? It's clear that you need to use cheaper spells, but only when there are enough of them.
The same level 3 magic for Nature or Life can perform much better than for Chaos, but this is all subjective. But in my opinion, a summoner is still more useful than a sorcerer, and their creatures deal more damage if there's nothing to eliminate them.
Chaos seems too easy, especially against the AI, due to its simplicity. And it hardly shows itself in defense, since all the buffs are for offense.