To clarify, that graph is specific to Ifrit only. It does not represent all R58 mobs, and definitely not any normal R58 mob.
The balance of that curve is fundamentally based on the mob's ATTACK POWER, and NOT their dLVL. Ifrit is an NM that has an extremely high attack over most R58 mobs, hence it takes more DEF to reach the supposed damage floor. This is why the curve looks so harsh. Increasing dLVL actually makes the curve LESS HARSH because the per-point decrease in damage taken for each point of defense increases with dLVL. With this in mind, dLVL actually helps you reach the damage floor. Again, it is the ATK of the mob that causes that curve to look so daunting.
There are a number of arguments you can make for or against dLVL, but that defensive efficiency curve would probably not be the best example to cite. I think you would make better cases for critical potency maybe. I'm usually very hesitant to post my opinion of mechanics like this because the answers are very complex, but I thought I'd come in here to clarify the actual mechanic for you guys since it's fairly confusing. But for the record, I'm a huge fan of dLVL.