Alternatives to "nerfing" ninja

[ChaozK]

They're removing positionals and buffing attacks on DRG. So why do NIN need to be nerfed after that again? Because these changes alone will take DRG higher than NIN in damage, and possibly have them compete with MNK for top DPS. There's no point in nerfing NIN if they're going to buff DRG, it'd be like nerfing BLM while buffing SMN, it makes no sense.

Because the devs probably think NIN is too strong in comparison to the whole roster of dps rather than just DRG is my guess. If it was just about DRG they could've just buffed DRG but since they are going the route of nerfing NIN, it seems to suggest they are unhappy with where NIN stands in the dps rankings.
Not saying thats actually the case or my own opionion but thats probably their reasoning.

Edit: Another guess is that they didnt want DRG to be close to or on par with MNK but would've needed to in order to make them as desirable as NIN. So they went the middle route of buffing DRG a bit and nerfing NIN a bit.