i just beat the demon lord with magic alone dude.
For moderate encounters you can run around zapping people with a quickcast thunderbolt
for boss encounters you put them to sleep and cast inflict wounds.
you can keep up the magic by withdrawing some from your reserves with the chant ability
Its totally viable my man. 10 magic is easy to obtain with a mage and that is all you really need. You got the chastity cage, the magic charm, enchanting your skirt for magic for an easy 3 magic bonus to stats not even including the perk points you can use.
I think it is incredible (not busted, I don't want majallis getting the wrong idea and nerfing mages again) for taking out all sorts of opponents. You can take down just about all opponents with it.
Also keep in mind that classes that cant go high with magic make up for it with their physical stats. You can keep the tempo going with casting sleep to then chant and reforge your gear to start attacking again. Also inflict wounds is a hell of a spell which rewards high investment into magic ability.
Also if you want to draw more mana that is what you use your perk points for as opposed to physical stats with a mage