And there goes both your credibility and your point: the complaints are that destruction is broken on its own (with utility/defensive trees). Yes, you can use conjuration, we know that. Now try it with no summon on master difficulty and come back to tell us how that went.
Inbefore "lol use all your magic!". People want to play a nuker type character, not a summoner. Having Atronachs completely changes the gameplay. Especially when they do more damage then your spells.
Also, keep in mind that the main problem is one of scaling. Naturally, you are at the very beggining of that problematic area (lvl 40+), so it is normal that you dont see much impact of it. YET. <--- keyword.
He hasn't lost any credibility at all, who the hell would use destruction by itself when there are 18 skill sets at your disposal? The game has not been designed to allow you to use single skill tree builds and be efficient. It's been designed around the fact that you will use multiple trees together each complimenting each other in various different ways. If you were to use one-handed or any skill in fact, by itself, with no other skill set, it would be just as weak if not less so than destruction.
As it happens i currently have a level 45 pure mage, and there isn't anything in the game, even in groups that can really cause me much trouble anymore. 2 summoned Dremora Lords and dual casting thunderbolts (which is great fun i might add) will destroy anything you come across with relative ease, (which is to be expected using skill sets that are capped out) this is on adept which is the game benchmark when testing classes. Anything other than that difficulty cannot be used as reference, as it is deviating from how the game is meant to be. The game should be balanced on normal for all classes with the other difficulties only used for more or less of a challenge, based on personal preference. Having played a pure mage i'd say it's balanced quite reasonably. The post level 40 damage thing is a myth, as i can still crush everything even without my summons, it's just more fun with them.
Also regarding your scaling issue, there is no true level scaling in this game, i keep hearing it over and over again on these boards, but nobody seems to know what true level scaling is. Oblivion had it but this game does not. Every creature type in the game has a set level, set hp's, set damage etc etc. Variants do not count as level scaling. Otherwise that would mean every game in existence has it. If there is any scaling at all, it would be the guards but i would have to specifically check this.
Oh, and my summons, with the exception of twin Dremora Lords, don't do more damage than me per second. Not even close.