>I'll acknowledge that the probability of death increases more slowly after you pass 65 mph [1], but it still increases. Human life is not negligible just because you want to get somewhere negligibly faster (e.g. by a few minutes at best, if you're going 70 in a 65 zone).
By extension, you should take every back road while you drive, rather than the freeway, because there is a minute chance that you increase probability of human death by going 65 on the freeway rather than 35 on back roads.
There is existing reasearch which indicates two things: 1) Most people drive at the speed they feel comfortable driving at but some obey speed limits (even if they are comfortable driving faster) and 2) It is differences in speed, not speeding, which causes most accidents.
Considering these factors, speed limits can artificially increases differences in speed among drivers. When the federal speed limit law was repealed, many states who raised their speed limits saw decreases in both accidents and fatalities.
That's not to say speed limits are causing accidents, but I think it's possible that they could be in certain context. It's not as cut and dry as "speeding kills".