Also, even if the low average life expectancy was due to excessive mortality among children with chances to live up to 70 once one reached 40 not much different

Also, even if the low average life expectancy was due to excessive mortality among children with chances to live up to 70 once one reached 40 not much different than today, it still does not imply that cancer rates were lower. It could be very well that it is those people who would die as newborn or children without modern medicine and access to clean water have bigger chances of getting cancer with age.