I understand what you are saying. However, I do think it may be a matter of semantics. No one really believes that children always outlive their parents, as if it's some kind of law of the universe. That's just a wish. It would be more literal to say that we hope that children always outlive their parents, but know all too clearly that it's not true. No one wants to see someone else suffering their worst nightmare.
I also don't think it was any easier (or less of an emotional burden) in the past to bury a child, nor do I think that there was any greater support network for it just because it happened a lot. In times when we didn't have names for diseases or understand why people died, and all deaths were blamed on curses or God taking them away for some reason we're not allowed to understand, the emotional blow and isolation from that had to be unbearable.
My great-grandmother buried five of her twelve children. Three of them were under the age of 1. One was almost 10. And one was killed at war at the age of 19. Can you even imagine? FIVE children? For a woman who was a homemaker, isolated, alone, no counseling services, expected to "get over it" and carry on for the sake of her other children, in a church who only told her that her babies (the baptized ones, anyway) were in heaven and that she should not question but only accept God's plan, and be sure to have more babies right away? (but if she wavers in her faith and questions God on the deaths of her other children, she'll probably have to bury those babies, too.) How she got through that, I'll never know.
It's easy to look at the statistics of infant and child mortality in times past and see them as only statistics, and forget that each of those numbers was a deeply personal and painful loss for each person who experienced it, and most of them didn't get help or support of any kind. It's easy to say, well, it happened a lot so people were probably more accepting of it and used to it than we are now. And again, I say, now, as ever, children dying is a headline every single day and there is no getting used to it.
My cousin just buried his 4 month old son early last month. I still say that no parent should have to go through what they went through. What I mean, when I say that, is that they are living every parent's worst nightmare, and I hate to see anyone suffering in that way.
When speaking of/to a grieving parent, you can say "no parent should have to bury a child." Or you can say "People die every day, and some of them are children, so statistically speaking, it had to be someone. Sorry it's you." Both technically true and saying the same thing, but one of those is going to get you kicked in the teeth.