You'll be able to repeatedly cast the spell forever, but the DPS isn't as high as a melee- or archery-focussed player can get, even without the ridiculous Enchanting/Alchemy exploit.
Fact is the DPS of the best Destruction spells is not high enough to take down high-levelled enemies. The spells are available at level 30ish onwards - at which point they're viable - but by the time you get to level 50, the enemies have gained 20 levels' worth of damage, defence and HP, and you're still stuck with exactly the same DPS. At that point, it's ineffective regardless of how much magicka it costs.
That's not true.
-I went to Fellglow Keep.
-Got to a part with an ascendant conjurer. Not sure how many hits this one took. I was spamming fireball while trying to dodge ice storms.
http://cloud.steampowered.com/ugc/649871686493229855/24823ABB80E0C33A5FC58A7E6F35F2BFA0EF6677/
http://cloud.steampowered.com/ugc/649871686493234548/21619F8B6D704E3358A7132DFA57ED0A5C268D37/
-Fought an ice wizard. He went down in 9 chain lightnings.
http://cloud.steampowered.com/ugc/649871686493749395/B4AE7A82C30915205685B730D389AC95A7B4C9B7/
http://cloud.steampowered.com/ugc/649871686493751469/E41185547D506C0C36BB9E36C1549D78A727E590/
-I got to the same part with 3 mages. This time it had a storm wizard. (apperantly I forgot to snap a photo after I killed them.) Killed all of them in 9 chain lightnings.
http://cloud.steampowered.com/ugc/649871686493750655/54B3FC5484E221909D95478A16DEF09C41541DAF/
http://cloud.steampowered.com/ugc/649871686493750248/CBA8FC9CDFF60EECB70255050AEE59CB94B0DEF0/
-Made it to the caller, and killed her. Couldn't see how many fireballs actually hit because of the teleporting and fire.
http://cloud.steampowered.com/ugc/649871686493750985/1C5A94F019380B097A4AEEF1ED6924A4D2CBB4A3/
http://cloud.steampowered.com/ugc/649871686493749826/E6638EFEDCC3752564CACF6633E24C75CBEE5E7B/
-Impact can stunlock enemies. Killed this dragon before he could even fly off of this rock. Unfortunantely it glitched so it didn't die.
http://cloud.steampowered.com/ugc/649871686500912127/D6B3E9D16A8BBBAB3ED5AA69C8D580F619C4595F/
http://cloud.steampowered.com/ugc/649871686500912643/28C7908BE2EF2C273275666383F400A413EBF641/
This is on master difficulty
-This is how I perked my Orc.
[img]http://cloud.steampowered.com/ugc/648745380544210553/2FB6141DEDF4891FCA6504815D03793439542CD0/[/img]
[img]http://cloud.steampowered.com/ugc/648745380544209333/924C9A9BC3C4020ADAEAD014C74042B11B8D86E5/[/img]
[img]http://cloud.steampowered.com/ugc/648745380544211208/A3E4684E4C98649D371A45CEDBBDD412A54F5E4C/[/img]
-Here's my gear.
[img]http://cloud.steampowered.com/ugc/648745380544211728/7AB89F5C94EEBFE1E32A3E461392841C46ABDFF1/[/img]
[img]http://cloud.steampowered.com/ugc/648745380544222308/6DD080D2145D66F7B9B5EC471662AE628898C898/[/img]
[img]http://cloud.steampowered.com/ugc/648745380544212989/B85642DAFF10F396220C77C0E6DC586CC897B4A6/[/img]
[img]http://cloud.steampowered.com/ugc/648745380544210173/BCBA5951A8D1A57595A40282BC9F30CCE192E0E3/[/img]