A couple of things I would like to point out. First one handed requires you to spend more points to increase the base damage. 5 compared to destruction's 2 if you stick to one damage type.
Secondly all destruction spells are ranged as far as I know. The damage you deal with destruction generally occurs well before your opponent is able to start dealing damage to you, assuming they're melee. There's also a perk that staggers enemies when you dual cast a spell on them, further increasing the time (indefinitely with potions?) that they are unable to deal damage to you. With one handed there's no way around getting up close and personal with whatever you're trying to kill so you are more or less taking damage the whole time.
Edit: One last thing. Some destruction spells can damage more than one enemy at a time. Under the right circumstances the total damage could be quadrupled or more.
It seems to me destruction isn't about how much damage you deal but how that damage is dealt. You have a lot more options with destruction, letting you decide what spell to use for the given situation. Chain lightning works great for crowd control or striking an enemy around a corner. For more up close and personal battles you may be better off casting one of the cloak spells and go on the defensive, downing potions and poking them with a dagger or something while they take damage from your cloak. You really have to use your head if you go with destruction but it can be devastating in the right hands.
I never said the numbers were wrong. And I never said that destruction wasn't unbalanced. But nothing I said was wrong either. My current character is more or less a spellsword and although the damage he does with his sword is much greater than with destruction, there are some enemies that it's just plain easier to take down with destruction.
You ever try going toe to toe with a giant? Let me tell you, you don't want to be anywhere near them when that club comes down. It may take a while and require some careful maneuvering but you can kill a giant with destruction spells without too much trouble, even at a low level. I cannot say the same for one handed or two handed for that matter.
There are several issues you failed to mention here - damage inferiority compared to weapon-using builds aside.
1, Most destruction attacking spells are ranged, and its true dual hand casts cause enemies to stagger. But to keep them in place you got to dual cast constantly, which cost truck load of magicka for not-so-obvious damage benefit. You can afford them come closer once or twice then use the knockback shout, and thats it. Potion chain chugging is inevitable for any fight longer than a dozen cast, it takes more pot for a mage than a melee-oriented char whacking their way through mob swarms.
2, Destruction damage doesnt scale as well as weapons. You cant pot every fight obviously, so +% damage is the only bonus you might have vs perk+poison+enchant+smithing bonus on a sword.
3, Higher than adept spells are almost useless, esp. mastery spells. There aint many packed mob group in the game. The spells are all AoEs, which cause trouble to your tanking follower in closed enviroment of this game's dungeons, or take too much time to cast. And if you choose single target spell only, you are stuck with adept-level damage output.
4, Defend is the biggest issue right now: to sustain reasonable rate of fire you have to invest quite a lot into magicka and +% regen, so your HP is pretty pathetic (translate: 1-2 shot by random mobs and 1 shot by bosses). You basically cant risk enemies coming into melee range, spell cloak is useless. If you build enough HP to make use of cloaks (even coupled with alter +% defend perk), you will be magicka-starve constantly.
Its obvious you have to combo destruct with other shools, but as it is, destruct provides so little DPS output to balance out its lack of utility. (hint: conjure's pets!). Single-player game or not, Bethesda should ve spent more time balancing gameplay so players wouldnt feel frustrated after committing so much time into their characters.