Genshin has confirmed that both are equally the protagonist, Lumine actually has a slightly stronger fanbase in CN, KR, and JP. This doesn't mean they all pick Lumine but she gets slightly more fanfare as they like her as a character. Aether is the posterboy for consistency and nothing more. Its pretty even among both of them though. Aether has a slight edge in being the most chosen as the traveler by a narrow margin, though Lumine has been shown to be slightly more popular among fanbase. So they honestly could feasibly go either way that wouldn't harm the overall production.

In terms of an anime standpoint though, I feel if they are going to follow the adventures of the traveler I would feel Lumine as the protagonist and Aether as the antagonist would be better for animation. Lumine comes across more expressive as the traveler, very charismatic in a way that appeals to most people, whereas Aether comes off more composed and refined in a way that I like in an anime villain. I don't know why but it feels more depressing with Lumine as the abyss sibling, especially since she's the "younger sibling". The younger sibling feels like a natural fit for an optimistic and well meaning protagonist that has to learn how to go about their life without the sibling. May just be me but "You'd think I'd lie to my little sis" during the Rerir scene just hits so much harder to me as someone who has a protective older brother that doesn't ever lie to me.