World first kill in t5 has a MNK doesn't mean MNK is neccessary. MNK can be replaced by a DRG. But bard and Blm can't be replaced by a MNK.
'King' means your DPS is way above others but like you said you just 'very often' and for single dps only. if you look total dmg in turn, you will way behind blm. That 's not enough as king. DRG DPS is pair with MNK. So MNK basically is nothing but a melee DPS and can be replaced by other job.