I have no trouble accepting that mana exists in quanta. What troubles me is the idea that mana clusters into meta-quanta like spell slots. Also that a spell like, say, Fireball, is totally unviable except with a specific meta-quantum of mana, at which point it does 8d6 damage.

Shouldn't it be a simple matter to cast a weaker Fireball with less mana, or a more powerful one with more? I get that you need magic to summon fire, and a certain amount to summon 8d6-worth. But all or nothing? Why shouldn't a 1st level slot summon 1d6 with a 5ft radius, a 2nd level slot 4d6 with a 10ft radius? One would imagine a gradient, even quantized, between Fire Bolt and Fireball.