That is such an annoying argument. Birth doesn't need medical intervention if you are discussing the success of a species. You're completely right, we won't go extinct if an avoidable number of women and babies die in childbirth.
But most people have a lower tolerance of loss than nature does. Most people want their loved ones to survive labour and birth, even if humanity would carry on fine without them.
Humans also don't 'need' cancer treatment, vaccines, or any of the treatment offered at A&E departments. We wouldn't go extinct as a species if we started leaving victims of road traffic accidents at the side of the road, or shrugged when someone with asthma struggled to breath.
OP, I don't think we should criminalise free-birthing (though I personally think it's an incredibly stupid choice to make). Women shouldn't lose the autonomy to make decisions over their own bodies when pregnant, and it might prevent women seeking help if things go wrong. I do think we should put free-birthing advocates under tight scrutiny and bring legal action if their influence is found to cause injury or death though.