I'm sure Square Enix, Yoshi-P, and all the other Devs get this all the time, but let's be real. If they were adding a set of gear/appearance items into the game for free, such as, apart of a patch, that's fine, who cares if it's dyeable or not. But, if someone pays real money for appearance items off the MogStation, then it should be dyeable. There's no excuse for it.
I know we might get the "It's a lot more work" or "We want the outfit to stay true to characters design." But PUH-LEEZ, It's not much more work, because people make mods for the game to have non-dyable items be a specific colour that they want (Here is a mod that changes Lyse's coat to be any colour you want: https://www.nexusmods.com/finalfantasy14/mods/20 ) and if they wanted the outfit to "stay true to the character" then other races wouldn't be allowed to wear those outfits.
I know it's a bit more work for the developers, but let's be real; it would sell a lot more outfits on the MogStation.