(sigh) again...
I play pure casters. I speak from experience.
Having 0 cost in ONE school does not mean that magicka level ups are wasted because you will almost certainly have to cast spells from others schools as a pure caster.
Melee has no more complexity or variety than magic. Simple. Different play styles and preferences, but one is not inherently more varied than the other.
Destruction is viable without using Enchantment or Alchemy just as One Handed and Two Handed are viable without using Smithing. Actually, with Jewelry, Smithing could also be under pure casters, of course. Depends how you want to play. Beth lets you make the choice. They just need to add Circlets to the Smithing of Jewelry (hello, Beth? duh!)
All that Enchantment or Alchemy do is make things easier. Nothing more, nothing less.
Of course, if you want to cast Master level Destruction spells, you will need Enchantments, although you don't need to make them yourself. However, you do not NEED to cast Master level spells to be viable and survive with no problem. Likewise, you do not NEED to have Restore Health or Restore Magicka potions that do 200+ pts per swill, either, nor do you have to have super versions of the regen potions. All of these options simply make things easier, but are not needed/required by any means.