Mages are not horrible. I'm a level 40 pure mage and I'm kicking ass. I can down an elder dragon without ever getting hurt or running out of magicka.
If you're having so much trouble with a mage, either turn down the difficulty or change your playstyle.
I find it funny when people's solution to a skill's abilities not scaling properly is, "Turn down the Difficulty" instead of, "This is broken and needs fixed."
Of course mages don't have combat prowess, they're MAGES, not warriors. They're squishy and die easily in melee combat. That's just the nature of being a mage, and that's how it's always been in just about every fantasy game. Build up your magicka pool, learn lots of spells, use more than one type of magic, keep moving (don't stand still, use Conjuration to summon critters to take the heat, use Illusion spells to disrupt and confuse the enemy, use Alteration spells to protect yourself and eventually paralyze the baddies, use Restoration magic to heal yourself and protect yourself from enemy magic with the Ward spells, and lastly use destruction spells to blast the crap out of them. And always always always have plenty of magicka and health potions available.
I think you mean, "Use destruction spells to make enemies laugh at you." I find it hard to believe you're 40 and finding destruction useful unless you're on novice, and even then it's a stretch. The hard capped damage of destruction means the game, health pools and other forms of damage will keep scaling well beyond the point at which destruction stopped dead with no way to further improve it.
The trick with destruction is to not always use the "most powerful" or highest level spells. I'm level 40 and have had Incinerate, Icy Spear and Thunderbolt for a while now. But I still always use the lower level versions, Firebolt, Ice Spike and Lightning Bolt. Why? Because they cost barely any magicka, and you can dual-cast them all you want and stagger enemies with the Impact perk.
Because people enjoy constantly staggering opponents to make up for the fact that the core function of Destruction, "To Destroy, to do Damage" is lacking terribly. I now find it even less likely that you're 40 and on a decent difficulty setting, and doing decent damage to opponents, if you're using that tier of spells because even the Master Level spells do too little damage on real difficulty settings in the high levels.
I almost exclusively use Lightning Bolt for everything, since I have both damage perks for lightning. I just keep dual-casting lightning bolt at an elder dragon, and it constantly staggers so it can never use its breath weapon and eventually goes down and I still have magicka to spare. But if you try to use Thunderbolt or something all the time, you'll only do a fraction of the same damage before running out of magicka. Wait to use the high level spells until several levels after you've gotten them, by that time you should have enough magicka.
Eventually being the operative word here.
Honestly, the fact that you think you need a large Magicka Pool at all tells me how little you know about being a Mage in this game. You only need enough to support your more rarely used schools, probably 500ish at the most if you've perked right. Why? Because the one or two major schools you'll be using? You can make them free.
Entirely free.
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Abusing the fact that you can infinitely stunlock things to make up for Destruction's damage not scaling properly to content, if anything, is an argument for why Destruction is so utterly in need of major changes.