Well, the Night elves have highborne - ofc, it's part of their lore, it doesn't mean the Nightborne are entitled to have druids or Moon Priestesses just cos they're night elven based, and because they are sub-race they don't need to have the whole spectrum of the night elves, Some might feel it is bad enough they have the arcane legacy of the Night elves, let alone give them more... but I agree with you.
I don't view the allied races as necessarily being locked to one thing or aspect of a core race or even having it exclusively at the expense of the core race. IF the Nightborne are a night elf off shoot, then they have value to the horde with Kaldorei like druidsm (in their own infelction of it) and Elunism too. Since blizzard love the horde so much, themes like Elune, the Emerald Dream, the Kaldroei civilization and culture may actually get a lot of updates and progression now that the horde is involved. They tend to ignore a lot of the alliance races in favour of Humans, but night elves often end up in novels and books, perhaps they feel guilty of showing more in game because they're only in one faction.
Maybe the Nightborne going horde could be a huge blessing in disguise for the night elves. Even though I personally hated it, it could actually drive night elf lore and development immensely. Already having Suramar gives us a good picture of how the Highborne and Moonguard night elves must live, future night elf cities would undoubtedly have that style and architecture.. and as the night elves are core race, it is likely to be driven from their end, but the Nightborne given a good share too because they are related and the horde have access to it via them. This is how it worked out in Naz'jatar - the ruins of Zin'Azshari had the horde able to partake with Thalyssra, which was far more appropriate than using a blood elf. And while I feel Jaina should have been replaced with Mordant Evenshade or even better Prince Farondis (it's the second time they've chosen a well known human over promoting the more appropriate Highborne character), I think it is already beginning to benefit the night elf group of elves a lot more.
Night elf lore is big and very interesting, and good, it's one of the most unique parts of the Warcraft races, and rally special, it really deserves to be one of the foremost and forefront races, and having them in both factions for now might help that cause immensely.