Earlier quoted context omitted.
For "a shitload" of years infant mortality rate was >15% too. How things used to be is not a good indicator of optimal approaches. A scientifically-minded person should not prefer random internet commentators to doctors' when the sole argument proferred by the former is that things used to be a certain way, therefore it's ok.
Probably not because of co-sleeping, many researches think that co-sleeping shows a correlation without actual causality.
It would be hard to study causality in this instance because of medical ethics. You cannot assign a treatment that you believe to be more likely to cause an infant to die, so RCTs are impossible in this domain.
http://www.medpagetoday.com/Pediatrics/GeneralPediatrics/164... -- co-sleeping babies are 21.7 times more likely to die from SIDS than non-co-sleeping babies, according to this study. That definitely doesn't prove causality. But the correlation is certainly waggling its eyebrows suggestively in causality's direction.