Your last line is why both of your arguments are rather pointless. You can’t argue Horizon is popular with or without Aloy because, outside of Call of the Mountain, every Horizon game has been centered around Aloy.
You can’t prove she’s the cornerstone of the franchise’s popularity because there’s never been anyone else carrying the torch. Likewise, he can’t
disprove Aloy’s potential popularity for the same reason.
Bringing up crossovers isn’t a strong argument for his case because while she may not have been well received in Genshin, she was well received in Monster Hunter World.
Until we have a full Horizon game without her, no one can say either way how much Aloy herself contributes to the franchise’s popularity.