We are already super great at debuffing.
Not really. Only debuff we have unique to us at the moment is Addle, and if the DATs are to be believed, other jobs will be getting it as the cap rises. BRD, without any JA use, has a more potent Slow. BLU has some debuffs we don't, a lot of them also being AoE or Conal. BLMs were always better sleepers.

Honestly, I'd be willing to drop the melee crusade if SE really embraced the enfeebling role, but that still wouldn't help most basic scenarios like EXP (mobs die way too quickly to get the full spread, nevermind MP costs to do so every mob) or seal farming where people believe slowing down or inhibiting a mob's attacks is a detriment to a MNK's counter/kill speed.