“Natural selection can allow a population to survive if a condition doesn’t kill them until they’ve passed sexual maturity and can produce children and live long enough to get them sexual maturity.”
True, but it’s still a condition that impacts the fitness of the population, so a population who didn’t have the mutation would have outcompeted us for resources and become dominant, rather than the other way around. That is, unless the detriment is easily overcome by changing your diet or something like that.
I just don’t find it plausible at all that such a detrimental mutation would become universal, unless there is some other, even greater benefit that the mutation provides us, or the detriment is easily negated. You assert that it’s basically impossible to negate the detriment for most of human history, so unless there is some great benefit to the mutation, that assertion doesn’t seem sensible.