They can either remove piercing from the game completely and adjust potency for the three jobs that need it. Or give all three of them their own piercing buff (just like slashing). Because of the reason others have stated, BRD/MCH are the only jobs that depend heavily on a single job offering a unique buff that they need. No other job in this game needs piercing except them.

As long as BRD remains meta, Crit buffs (Chain Stratagem, Battle Litany & the Spear) will always be wanted. BRD dps depends heavily on Crit. You can do runs with the same gear, same group composition, yet end up with completely different numbers each run because of Crit (Damage variance on BRD is very huge).

Also don't forget that Battle Litany affects both dealing damage and healing (Make SCH happy with Crit shields). I don't see DRG losing its spot in meta even without piercing unless another job is introduced offering Crit buff + better raid utility than Dragon Sight.