Ideally, the DRK will need to be able to fit into raid compositions without being oppressively overpowered, but also not be extremely weak to the extent that everyone will continue to run PLD/WAR compositions.

There's already a dynamic where 3 roles fight for 2 slots in a party: Melee dps. Post-DRG buff, each dps can do comparably high numbers, but you still see many progression groups prefer 2 of the 3: MNK and NIN. Despite the very good buffs and QoL changes to DRG, they are typically still 3rd preference due to the fact that they don't really provide much utility. MNK has Dragon Kick, while NIN has Goad and Trick Attack.

In my opinion, what's going to make or break DRK in progression raiding is not necessarily the raw stats in tankiness and dps, but what kind of party utility they can bring to the group. The reason almost all groups run WAR/PLD instead of WAR/WAR or PLD/PLD is that each class brings something that the other does not (Storm debuffs for WAR; Halone STR debuff, ability to silence and stun lock for PLD). We'll have to see what utility DRK brings to the table to warrant subbing out either WAR or PLD, and I don't think its going to be something like "It's tankier" or "Deals more dps."