Easy, you take a tank with you and make sure they hold aggro... on top of that, there are multiple mechanics that have you stay in melee range (eg. Shiva's Whiteout, A10S' group shared attack in Triple Charge, Sophia's Thunder III, among many others), so it's kind of outrageous to say "What is a caster that attacks in melee range going to do if they have squishy gear?", healers are even squishier than casters, or even as squishy.
The Earth will continue to rotate, as usual. And Red Mages will continue to live through melee range, or caster range, as long as they aren't eating stupid attacks because of a tank or their idiocy in thinking they can survive a certain AoE.
