First: congrats to Horizon to achieve that remarkable milestone.
To your claim: "That's right, this means that Aloy has now officially taken Lara Croft's title as the most visible and iconic heroine in gaming!"
I disagree. You take the most successful game each and declare that makes them an icon. But in actuality an icon is derived from ongoing experiences. James Bond is an icon, because there were movies over the course of more than 50 years. Some bad, some good, but there were always some. A single movie that in itself is very successful doesn't create instantly an icon. So no, in my opinion Aloy hasn't replaced Lara Croft as an cultural icon. It is possible that she will take that spot - if for a decade or more she is featured in more games, novels, movies and so on and stays relevant. We will see if that happens or Sony moves on to drop Aloy and replace her with another IP.
In my opinion an icon shows stability over the years, because it is more than a short-lived trend. If a considerable amount of people still read the books/play the game/watch the movies in 10, 20, 30 years from now, then you have a cultural icon.