And if all else fails, she could still cast another cheap burn spell right after the first if she had excess mana left in her system.
Right....and so mana burn doesn't matter.
"Doesn't matter" isn't the same as "it's okay to pretend it doesn't exist anymore without in-universe explanation". As I said, they can tip-toe around it and just not bring it up for all I care. Bringing it up again would make me happy, but I don't actively demand it.
Like, in what instance (story-wise) do you see Mana Burn coming up where the answer isn't just...cast a smaller spell right after the big one?
Candra is exactly the kind of character who
does throw around spells in quick succession (or could, if necessary), and casting another spell with white mana instead is what Jodah does when he fails to open the gate to Phyrexia. But to answer your question, mana burn could come up when
- an unexperienced mage simply doesn't know what they're doing and screw up a spell
- someone draws mana from a land whose colour has changed since (like Jodah does when he gets mana burned before then casting a white spell)
- someone has messed with the local mana (e.g. by diverting mana lines from a nearby land) and it produces a different colour now that the character can't handle
- a character gets knocked out/injured/distracted while preparing a spell, so now they've got mana in their system but can't mentally finish their spell (like what happens to Jodah in
The Shattered Alliance when he gets stabbed in the throat while preparing a spell)
- a planeswalker is on an unfamiliar plane with weird, volatile or overabundant mana like Zendikar or Shandalar
- a character cast a spell or curse on an opponent so they lose control over their mana pool
Of course, your mileage may vary; some of these situations could be resolved by just casting another spell, but that doesn't mean characters won't get mana burned a little before they manage to cast a spell. Including mana burn in a story doesn't mean someone has to
die from it or turn into a zombie, but it would definitely serve to create some drama and suspense (like it does in the Jodah examples I brought up).