Honestly, I play a melee character.
Try playing a pure mage and come back to tell us your opinion.
its not possible to go into combat without a companion as a pure mage. While offensive magic can deal out hefty damage, it's simply not possible to
rely on it, or
rely on magic at all.
If you dropped all your melee weapons and only used fire, the first enemy that you fail to kill with your first destruction burst would lodge an axe in your face, and there is no way to avoid it. I have 400 magicka and lots of destruction perks, I started a dual-weidling character to comapre, and its not een funny how easy the game is in stealth or fighter mode.
Conclusion: Magic is a very effective sidearm, but it simply cannot be
relied upon in combat as a primary weapon, you have
no fallback.