I disagree, actually! For a job which relies a lot on buffing allies, I would consider how it feels to play alongside NIN just as important as how it feels to play NIN. I think this sort of thing should be considered for all jobs, to an extent -- for example, Black Mage wants to stay still as much as possible to keep uptime with its casts (and remain in leylines), but it is given a myriad of tools to allow it to move so that it's not burdensome for either the Black Mage or its team to play alongside it. Even so, the limited mobility of Black Mage is something I would take into account, relative to the higher mobility of Summoner or Red Mage. This is just an example, however.
I can understand why it might make more sense to prioritise the gameplay feel of a job over how it impacts other classes, and I would generally agree with you that when changes are incoming, the preservation of the gameplay feel of the job itself takes more precedence over how it impacts others. Despite this, I can't agree that in a team game, designed to play in groups of 4 or 8, "how it feels to play alongside X" isn't important enough to consider. I think that the interactions between different jobs with their various quirks is one of the many things that makes this game incredibly fun to play.
I would also have to disagree here, though we're definitely arguing on a shaky foundation, given we don't know the exacts of how these changes will shake out. Despite this, there is a definitive 'best time' to activate party buffs in the opener, which is exactly where we currently pop Trick Attack. Popping the Mug buff earlier (to leave the current opener unchanged) would desync that buff from other party buffs. Embolden, for example, is 20s, and is generally popped at the same time as Trick. This would also do nothing for the fact that Ninja currently struggle to fit its entire burst into the 15s window on odd/TCJ windows. I would imagine that Mug and Trick would be double-weaved together to ensure alignment with other party buffs. Extending the duration of Mug doesn't really impact when we pop it, because we will always want to synchronise with other party buffs as much as possible, givne that they're multiplicative. Searing Light is, I think, the longest party buff at 30s (? not 100% on this), so at the very least we would want to pop Mug at the same time as Searing Light, to maximise their overlap.