is all TES games caster skills scale by alowing you to cast more, as in more skill = less mana cost.
spells are base line more powerfull than bows or melee, and limited at start, heavily by their high mana costs, you use gear and skillup so you can chain cast the more powerful spells.
I know how TES games work (and don't work), and that is the problem. This skill doesn't work like previous games.
When I say that raising the destruction skill make no difference, I mean it. It has no effect on damage..AT ALL. The effect it has on mana consumption is so negligible, the difference can't even be noticed between 10 and 100. There is no reason to raise the skill other than perks.
I'll test this (again) when I get home, but I suspect if anyone here who has a high level mage were to use the console to lower his destruction skill to 10, yet keeping everything else the same, he would notice no change with the performance of his character. He might notice a slight decrease to mana efficiency, but that is it. He will do no less damage and will be equally effective in combat.
The skill only exists to limit which perks you can buy and which spells you can purchase. It seems to provide no other function.
No other combat skill in the game is like that. Even conjuration scales, since the level of your summon matches your characters level.
And regarding gear upgrades, there aren't any. Not for damage. Around 80-90dps is the highest damage you will ever do in the game. EVER. This works fine until you get to around level 40, as other have said. Then you will encounter creatures with 1k-2k health that can kill you in seconds. Then you have to fall back on another skill. Destruction loses its purpose, because its ineffective and other skills are blowing it away.
Using your summon as a distraction is fine. But when it is doing 5-10 times as much damage as you are, it becomes pointless to waste your mana on destruction spells at all. All that time and energy wasted on something that could have gone to a skill without an expiry date.