Not a great analogy because a druid has proficiency in medium armor other than hide, while a champion fighter doesn’t have a spellcasting feature. It would be like if fighters learned spells, but then in the wording of their spellcasting feature it said “champions will not cast fireball” despite it being on their spell list.
I think that’s one of the biggest sources of contention here. WotC could easily have given druids proficiency with light armor and hide armor if those were the only armors they wanted them to be able to use. Clearly, since they have proficiency in all medium armors and we have confirmation that the restriction is intended only for lore reasons and not for balance reasons, it should be uncontroversial to say that druids can make use of chain shirt, scale, breastplate, and half plate made of alternative materials without upsetting the intended game balance.
I have no problem with alternative materials for armor if that's what the group wants to do. There have been non-metal heavier armors throughout history. It's not like D&D is particularly realistic when it comes to armor in the first place.