You just have to have enough pure healing required, in a small enough window of time, that tanks/DPS healing tools are not enough to deal with it in time. Alternatively, throw more Esuna-debuffs in there. Nobody's zerohealing TEA any time soon, because Throttle will take out 5 of your team (the only thing you can do with zero-heal runs there would be to bring 6 bards as your DPS, and then you don't have eg SMN phoenix heal or DNC curing waltz)

When things do 'big damage' but there's nothing beyond that, like a doom timer or whatever, then mitigation can stand in for flat healing. And with how much mitigation we have across every role nowadays, that leads to what we saw here. It's also part of the reason why SCH SGE is a viable comp, and is in fact 'the meta' in DSR. Because the amount of mit required vastly exceeds the amount of flat healing required.

As an aside, changing healing to be more Barb EX in nature (raidwides are far more common, but hit less hard), would vastly incentivize pure healers over bringing two barrier healers, while also allowing 2x barrier to be valid too. Picture the scene, you're a healer, you have 70k HP. You have 80k damage coming in from a raidwide. You either mitigate it enough to make it hit 'less than 70k' or you die. Now picture the same, but the boss does a Knuckle Drum style rapid fire of hits, where each hit is 10k damage, and takes 5-6 seconds to do the full 80k damage. If you throw out a heal at any point while the move goes out as a pure healer, you survive the mechanic. Barrier healers mitigate the 80k total down below 70k and live that way, same as now, but Pure healers are able to deal with the damage halfway through the move with their stronger flat HPS output. Proactive healing and Reactive healing in a sense. It baffles me that SE hasn't considered this yet, and I can only hope that come 7.0, they consider it after seeing the Barb EX reception