I think you may both be right.
Assume you start off with a small niche product and keep increasing your userbase.
Then the characteristics of the users at the fringes will change the more you grow.
That is to say, the former Marls in the middle are different (and likely not so shallow) from the next-generation Marls on the outside. Eventually, your notion of what is the average user, and OP's notion of what is the Marl that finally kills the UX, will align.
Then the characteristics of the users at the fringes will change the more you grow. That is to say, the former Marls in the middle are different (and likely not so shallow) from the next-generation Marls on the outside. Eventually, your notion of what is the average user, and OP's notion of what is the Marl that finally kills the UX, will align.