Destruction is def broken in terms of damage output. While it makes sense at lower levels, because it's capped it doesn't work at higher levels. People are complaining that smithing and enchanting breaks non-magic which is why it can't compare. That's not true. Here is a build that you can get from completely random loot at level 45:
You can easily get gear that gives you +100% damage, combine that with the perks for a one handed weapon and you have +200%.
At level 45 you can easily find one handed weapons that do 45 damage (with perks and gear) and 60 points of magic damage.
That's 210 damage per power attack. NO enchanting, NO smithing, random loot ONLY, and it's conservative. You can find gear that gives you more than +100% damage at that level, but I chose to be conservative. So compare that to the next most powerful spell? That's incinerate, doing just 90 points of damage with max perks. That's 2.3 times less damage, with maxed out perks. I think Bethesda intended you to use smithing so let's be ultra conservative and say you have smithing 45 with the double smithing perk. That's 282 damage per power attack.
Clearly magic is underpowered by a factor of 3. Which is what I would have guessed before running the numbers. Worse yet, most of the enemies you'll be facing at level 45 will be Draugr Deathlords, with 1400 hp and similar enemies. Those take 5 hits each with one handed, yet 15 hits with spells. Not only that but they take 800 magicka, each. At level 45 you will have no more than 400 magicka typically. You usually face a few of them at a time. Also you will miss a few spells, guaranteed so you will spend 1000 magicka per enemy.
So that means at base cost it actually takes 2 TIMES more magicka than you even have to finish off the average enemy with 1400 hp. It is not uncommon to face 4 of these enemies at once. If you have 75% spell cost reduction and 200% magicka regeneration, it still take you 1 minute and 15 seconds to finish off EACH of these enemies. It takes quite litelrally 10 seconds to do the same thing with one handed attacks, even without smithing.
To summarize:
Destruction with 75% spell cost reduction and 200% regeneration, clearly exploit territority, takes 7 times longer to kill a typical enemy at level 45 than a one handed power attack from a warrior with only random loot, no enchanting, and no smithing.
Personally I find magic to still be usable at higher levels. However only with extreme exploitation. The only thing that seems to work is to do the following: Slow time > Mass Paralysis > Blizzard > Firestorm > Firestorm > Firestorm >
Firestorm > Firestorm > Firestorm > Chain lightining. Even after casting 7 master level destruction spells back to back, enemies with 1400 HP are at 90% health so I still have to use the disintegrate perk from chain lightning to kill them. it is impossible to cast these spells without paralyzing everyone because I would be interupted. It is also impossible to paralyze without slowing time.
Edit: this is on expert difficulty, not master.
Draugr Deathlords, 1400 hp (I assume thats on novice, it will be equivalently more on expert) so that means it will take 7 dual casted incinerates to kill them (1400/220=6.4), so yeah thats not 15, they will also be immobile during these spells due to stagger. Incinerate by the way does 100 damage (if you have the illusion fear perk), so 220 damage dual casted. That breaks down most of your point to be honest. Look at my above post to see how much damage you can do with destruction potions.
At the moment you have to use enchanting to be able to cast expert spells effectively, they will hopefully balance this. The problem with destruction is the cost, not the damage.
The master spells do more damage than stated, this has been tested. I use the master spells with become ethereal or sneaking, allowing me to mass paralyse or blizzard to start the battle of, then I can just use chain lightning or the expert spells to finish. I usually fury the deathlords though, then just use expert spells and mass paralyse. It's not as hard as you're making it sound.