The change most likely is because they're trying to change what the game is.
Go back to the Live Letter with the 10 year Roadmap (think it was 68?). They don't want the game to be considered a MMORPG anymore because that has deterred many of the long time Final Fantasy fans from picking the game up. They want the game to be considered a RPG that can be played alone or with friends.
That means updating some of the more popular features to be friendlier to solo players while leaving group play intact. Relics appear to be one of the conversion tests. In the past, there would be no way to get a relic weapon at the intended level solo. By changing what was needed to Causality, which does have a couple of solo sources (Variant, level 90 treasure maps though the player would have to skip a portal if spawned) in addition to all the group content, a solo player could still earn their relic. It would be really slow but it could be done.
As I said, personally I don't agree with all the changes they're making but I'll adapt or move on. The move from MMORPG to RPG that is solo or with friends is one of them. It's a move that has an unpredictable outcome for the sake of people who, as you said, aren't even playing the game. Considering how much effort they've put into making content more solo friendly, it's unlikely they're going to revert the changes. I can waste energy fighting a change I'm not certain will have a neutral or positive effect on the game experience, or I can walk into it with an open mind to see what happens. I don't see this as a battle worth fighting. SE is going to do what they want to do and only reconsider if they start losing more players than they gain.
The value of glamour and other cosmetics is subjective to the individual player. You say that to you, the journey was the point. Would you have gone on that journey without relic weapons being a reward? That's what I did with Castle in the Sky. Expert crafting intrigued me and I wanted a long term goal to work towards. The mount was not my objective and the only time I use it is when I get Aswang as my SB weekly B rank (or when I'm chasing it around the Steppe to map Orghana) because I find it amusing to hunt Aswang while riding Aswang. If it wasn't for that coincidence, I wouldn't use it at all.
If we want a journey to experience, we do have some small ability to create them for ourselves even though this isn't a sandbox. I've got a friend who sets restrictions on himself every time he starts a new character. Maybe he's doing it semi-hardcore (no dying due to his own mistake), maybe he's RP walking everywhere and refusing to use teleports unless unavoidable, maybe he's doing it without any XP buffs outside of those he can't control. He's doing it for the fun of a different journey.
SE gave us all the chance to create our own journeys for this relic series depending on our personal preferences and it seems like almost everyone ignored the opportunity in favor of efficiency or some other excuse. So it goes.