My reward are the tomes for completing the dungeon, so yeah, whatever is in the chest is a secondary. The coil comparison fails for that reason, I am there for what's in the chest, not some other reward.
I guess it depends on what you mean by difficulty, but there is plenty of ways to make 4man content challenging. You don't need tank swaps and two healers to make difficult boss mechanics. Making it so new 4man content isn't instantly irrelevant could also help.
Last time they tried this with Pharos Sirius everyone cried horror. Also they nerfed the hell out of Mog Hard Mode. I am reserved to the fate that dungeons are mindless grinds at this point without much relevance aside from tomes or being recycled for relic.