You can have your cookie back.
You don't even know the true ideals of Balance if you're quoting that Legion made them more distinct when you're carrying a weapon named "Scythe of Elune", Talents and spells of "Fury of Elune", "Incarnation: Chosen of Elune", if that's not a slap in the face to say they are not ignoring the quests from 2004, you really need to build on your observational skills. Also Sunfire was added back in patch 4.0.1 not just recently and added to your Solar aspect in the older gameplay styles, they most likely changed it to Solar Wrath in 7.0.3 to make individuals more aware of it's place and does pretty much the same thing even prolonging your Sunfire. That's back in Cataclysm if you don't know it. That's 7 years before Legion too.
I mean if all of the outdated things you claim are there then why finalise things like aforementioned? Even previous expansions changed how Balance worked in-game, if it didn't fit anymore why did they still keep the Solar aspect, even at points work from Solar and Lunar sides with an Eclipse centre?
You may not know but Elune could have made the Prime Naaru - Xe'ra, not just a Naaru, the head cheese which if you didn't recall is the one that scolds us inside the Light's Heart and talks to us about Illidan's journey. Why am I mentioning it? Because it relates right back to Balance and those old quests with Elune favouring Moonkin. It pretty much could have made them all and not just the Moon Goddess we've been led to believe. If she can make beings of Light as well as be of the Night, it suddenly makes sense for Balance to use both Solar and Lunar magic.
And Blizzard don't really add sprinkles of hints to not add it in later, if you want to call Elune makings of Naaru a bluff. There's tons of confirmations like Chen's Old Keg quest in Vanilla for Pandaren to arrive in Mists (and they were meant to be in TBC before Draenei's came), a Worgen in Moonglade from Vanilla before they arrive in Cataclysm or best yet Timeless Isle with Garrosh and Kairoz literally right before Warlords of Draenor and time travel! There's so much more.
What's weirder still is that Tear of Elune... It somehow being all about the Night, Arcane and crap... It can awaken a Light being.
So she herself isn't going to be strictly stuck into the Lunar side of things when it comes to pass. Actually, with that in tow it makes Moonkin Form even more reasonable because if Elune uses both Arcane magic and Light magic. I mean they have used both Solar and Lunar energies since 2004 and with this new scope, it makes it oh so more understandable compared to your current understanding of only being Arcane. It states Nature magic but considering Fire and Nature are at odds... As much as Light and Shadow are, seems even stranger? And the patron to Druids isn't even Elune, it's her son Cenarius.
I mean, I know they may be outdated quests but it's sure weird to refer to it after 7 years later of lore and context that was added in in 2004, concludes to still being legit 13 years later as the theme apparently.
The entire class and the lore too that made the basis to it all which surprise, surprise they went back to for 'fully fleshing out' in Legion. They are trying to fulfil the original intention they had in the first place which is why they dropped that nice long quest (shown in picture above) actually in a class hall. To me it shows those 2004 quests were thrown in to make way for something, that the game moved from and instead the story of that area grew so large and awesome they thought "Fuck, let's make it epic". After all if Elune made the Naaru, her true nature has now been blow out of original perspective and ties in nicely with it being made more epic than it originally was all those years ago, with virtually shit technology, internet connection, resources and more.
It's quite funny you mentioned they fleshed it out but missed that Legion actually went back to older characters, times and lore to actually fully flesh themselves and amazingly not just for the Druid campaigns too! Let's look at Dis Priests, they've always been between both Shadow and Holy - Dark Evangelism was one of my favourite things about them, Shadow Priests have always been with the Void and not "finally" even NPC's were hanging with Old Gods as trash, boss encounters, quests and more... As for rogues it's about fucking time they got a class fantasy rather than being the stealthy, sneaky bastards and nothing more. Better yet, Thunderaan says Hi again from his murder back in Molten Core! Even folks like Meryl Firestorm, Arathor the Redeemed from other classes and many more which date back who resurfaced and have a cause. Heck even the existence of Suramar with Paladin Night Elves are even older then the MMORPG and made an appearance in their RTS games prior, only to be bought back into Legion!
Golly.
So no, they are not outdated as you suggest, Legion has in fact updated and reclaimed a lot of lore, whether in-game, out of game, in original RTS's and/or supposedly removed, whilst building on the very building blocks they've sowed for the 13 years of online gaming and even before that. Where was Aviana, Ashamane, Ursol included before Legion when you have instances in-game of War of the Ancients?
TL/DR:
I know you'll pretty much denounce all of this as it "doesn't matter" to you and that's fine. I mean it's so outdated they've built upon it and reclaimed it evidently.
But you know, at it stands currently in-game as Elune having a hand in both Arcane and now Light magic, it actually makes more sense for the Balance spec being able to use two sets of magic which Light and Arcane respectively as Solar and Lunar that form to be Astral magic. I mean this was all started back in 2004, with those silly quests, because after several years of gameplay, changes to them and such, they hung onto the base principles of Solar and Lunar magic, to nowadays, carrying around a Scythe of Elune because it shows she still favours them and gifts them in a way to use Astral power to them because after all she is stuck up in the sky, hanging in outer space being all about the Arcane and Light magic.
Under your guise of it being outdated then the class fantasy shouldn't even have the Solar half and be all about the Lunar section too. Afterall, Elune is only of the Night, a Moon Goddess and Arcane based to you. Why don't you just be honest and say I don't like being a "giant chicken" which is actually an Owl Bear, like the Moonfeather take for the Feral skin being a Owl cat, rather saying it is outdated and trying to justify it without even properly looking into the content you're playing.