You're thinking of the HP values we currently have vs the skills and how they work at 70. We'd obviously have higher HP values, and the usual item level sync would be in place, which effectively drops the high damage seen currently at even i320 and above.

As for burst damage? Healers could already deal with most of that (with the exception of BRD burst, which is still possible, but not a full HP kill). I survived double Fell Cleaves in Heavensward, even without mitigation. Fire 4 was always potent, even if it didn't crit, but that's why you never let a BLM freecast.. Kaiten Midare? Before, another SAM/DRG/MNK could stun that. They can't now. They can't at all.

Adjustments were needed. A complete scrapping of a system that worked and had its own checks and balances was not.