
Kids have infinite hit points and are easily manipulated. In a pure unregulated labor market, this drives out everyone but dumbest kids as laborers, and that's bad on many levels. Marking kids as legally incapable of labor and putting them into a 20-year timeout as they engage in educational busywork solves this problem.
Confession: never actually read Centuries of Childhood( https://en.wikipedia.org/wiki/Centuries_of_Childhood )
2. It's too strong of a claim to say that medieval or early modern warriors suffered from CTE (mostly because retroactive diagnosis has a terrible track record) but I think it's very reasonable to say that warriors of this period suffered from severe psychological and physical distress that derived directly from their training. Likewise, you're leaving out the fact that nobles hunted and rode on horseback (with no helmet!) on a daily basis and that these are exceptionally dangerous occupations by modern standards.
3. Smallpox might last a matter of weeks as an active illness. But it is permanently disfiguring. Elizabeth I is just the most well-documented of what we can imagine were literally millions of people who suffered trauma from smallpox scars. Similarly, many illnesses of the period (like malaria, polio, scrofula, etc) were chronic. As for gangrenous limbs, Henry VIII suffered from a wound (from recreational jousting no less) that ultimately killed him via a leg ulcer, but it took years of agonizing pain to do so. Maybe not gangrene, but the point remains that a world of sharp objects + no antibiotics is no fun for anyone.
4. It's very difficult for me to imagine that the pain of losing a child ever really goes away, even if you live in a world where it's commonplace. Likewise, the evidence about children not being considered "fully human" until age 1 relies on naming practices. But it ignores things like murder prosecutions for infanticide which were widespread in the period - if babies weren't considered fully human, why did courts care if they were killed? And finally, most childhood mortality occurred after age 1, regardless.
Edit: I mostly agree with your critique of the OP, by the way. But I think the fact that we agree on humans from the past being more or less like us conflicts with your points about parents in the past being relatively un-concerned about the loss of a child, etc.
In general I'd wish if people were more careful with general pronouncements like this — just because some people somewhere did something (say, hunt by running, eat only meat, eat only berries, sleep in two parts, sleep with the head hanging down, bake crackers, not cook anything, whatever) doesn't mean all humans did that until 5 years ago, nor does it mean it's a somehow best course for every single human dead or alive. Now, that it may be a good idea to try out (depending on specific idea, with specialist supervision maybe), but it might turn out horrible.
And I doubt anything like "optimal" sleep even exists, unless you start including additional conditions. You need work, you have to balance the need to sleep with the requirements of it (and some enjoyable jobs do start early), you might even a tendency to sleep too long for own comfort unless kept in check, and if you have apnea sleeping longer will only make things worse.
So, my annoyance here is mostly with the generalization made by the statement — not "we". Some other people, at some point in history, did, and wrote down all the awesome experiences they had while doing it.