I think you forgot something : despite the fact it's a mmo, FFXIV is a RPG and even more, a final fantasy games.
it have to keep easy content for story, that's why ultimate and savage are a thing, for player in seek of challenge outside the STORY content.
there is no need to make dungeons harder, you say it yourself : dungeons are ran by a majority of player and the majority in FF is casual and is not looking for any challenge + dungeons are part of MSQ or a side quest one but still, for story.
It is not a bad idea to focus the balance change and job adjust on the most difficult content, if you use NM or dungeons as standard for balance, you'll end up with unecessary nerf. That's why savage like any mmo with heroic, mythic etc is the standard used to apply change and balance jobs