Benelovent protection and Elain.
I think the three Archeron sisters represent a very deliberate progression in how ACOTAR approaches female empowerment, and Elain is absolutely necessary for that progression to work.
Feyre's story starts with saving her sisters and it shows what benevolent power can accomplish, but her sisters' happiness still depends on their place within a privileged ruling circle centered around Feyres husband and his goodwill. Nesta's story elaborates on the idea of power through her conflict with the Ic and her Valkyries, who show women gaining independence through cooperation rather than protection or romance Yet even that progress depends on access to the Night Court's resources. Nesta untimately saves Feyre in a very specific, meaningful way, making her equal to Feyre. You can even say they have now reversed the roles entirely, with Feyre indebted to Nesta, depends on how you see it. But the unresolved problem is that women's freedom still depends on their circumstances and connection to power.
That's why Elains pov matters so much . Feyre and Nesta have gained independence while continuing to treat their supposedly weakest sister as someone who needs protection. If Elain remains a supporting character, it underminds both of her sisters growth, or at least limits it greatly. If she just becomes another all-powerful heroine, it reinforces the idea that women must prove their strength to join the cool girls club.
Her story is already well positioned to extend already established themes beyond the flashy privileged circle: through the band of exiles, her domestic work, the human world and the servants so I really dont see why some people doubt she will be the next central charachter. I see why people assume nesta will remain a main charachter because of the hofas stuff, but Elain really needs to finally affect both her sisters. Feyre already stepped aside for Nesta so having Nesta become the next great savior would simply replace one exceptional sister with another. It would be equally ridicioulous if Elain ultimately became the protector. The only true endpoint is a balanced relationship and harmony betwen all three sisters. It would be the opposite of good storytelling if that was not the case. Elain desperatly needs to become equally consequential to their futures rather than remain someone whose future they both still determine for the grand narrative to work.
Funnily enough, the fandom's response to Elain just recreates the incorrect assumtions and unheathy dynamic the sisters started with. Her character and future are constantly defined through others' perspectives, while the ship wars largely revolve around what others want from her, despite her outright saying people dont know her.
I'm sure others have made similar observations. My point isn't that this is my preferred outcome, but that it's the only narrative progression that makes the previous sisters' stories amount to something beyond two individual success stories.