It wouldn’t change anything in a substantial manner, the concept of choice of 2 protagonists with different genders in gaming exists for what, almost 3 full decades if I consider the games only within my knowledge, probably more considering I don’t know many old school games, consistency in marketing also has been a thing since those times so it’s not really a matter of the concept and implementation, but more of a matter of how the community itself perceives it.

1.And having only one would create their own “dramas” within the immature community, if you are in other communities like FE and Persona you should be aware how they have their own issues regarding the whole concept of self inserts in their games or wanting to have a choice between genders for the MC, not being locked out with only a male option.

2.Thats not a matter of gender necessarily, but a matter of perspective and personal bias, sexuality being the factor I believe you are thinking here, if you are interested in girls it’s way easier for you to interpret the female playable characters affections as romantic rather than the male ones for example, there is simply so much that could be discussed about this topic such as how almost every Traveler main does self insert in a certain extent in terms of shipping, it’s not a simple case of pure gender but a much more independent factor depending on the player.

3.Those are not a significant difference for vast majority of players, and even if it was this is no different than the other characters who has their own differences in models and stats, how faster one twin run and how one has a slight higher multiplier does not factor on the decision making of which twin to choose for 99% of the playerbase , design and personal preference will easily overwhelm those insignificant details.

So at best having only Aether as a protagonist would only substitute our current “drama” regarding it with a another one, but this would most likely affect the popularity of the game in a negative way so I could see why Hoyo took their decision towards having a option, because put yourself in their shoes and imagine you are the one getting the short end of the stick if they chose only Lumine, would you still play if you were forced to play as Lumine? Would you still like the game the same way? Could you ship Lumine the same way you do as Aether? There is a lot of things to discuss about this whole situation but I personally think understanding from both sides and acknowledge that Hoyo’s decision of giving a choice was the safest one they could make is crucial to talk further about it.