I'd say killing a dragon on master difficulty at whatever level that was, is overpowered alone. One hit is just insanely overpowered.
That video kind of proves that with a proper playstyle (Moving around a lot, not getting hit, using all of your spells at the right times) you can be a very strong pure mage. I guess most of the haters will now come in complaining that you cannot be a pure destruction mage, to which we can all point and laugh since being a pure one-handed warrior is just as silly.
Being a "pure" anything is just silly.
That being said, it's quite possible to get by with one-handed as your only combat skill. Same with two-handed, or even archery and conjuration. But the same cannot be said for destruction...at least not after level 40ish.
The complaint against destruction was never that you couldn't win with just destruction and nothing else. The complaint was that destruction, as a skill, becomes largely obsolete after level 40, and there is no way to improve it after that. It really svcks to spend all the time, energy and perks leveling a skill, only to top out 10 levels below the theoretical cap. Watching the worlds inhabitants get stronger while your destruction skill stays exactly the same (and by relative effect, gets weaker) is very frustrating, especially if your chosen play-style has depended on destruction for all of those hours.
All these threads are just asking for a very simple fix to see destruction scale in the same way as all other combat skills. It's very interesting to ask for such a simple fix, only to have so many people respond as though you've personally insulted them in some way.