In other words, they fail to take into consideration that reduction of challenge (e.g. through technological means) is also change in everyday behavior that deviates from the "orthodox" way of living, but nevertheless it makes life much better. We don't live how our bodies intended to live, and prehistoric behaviors are no longer practiced - but the fact that there is no need anymore to exercise this sort of behavior is a good thing, since the necessity to do so implied struggle, discomfort and scarcity - all unfavorable circumstance.
In short - living as nature intended implied struggle, and the cessation of that way of life is in many ways a result of removal of struggle, and not necessarily just some "unorthodox" living that allegedly puts our being under unnaturally high amounts stress.
My point: Yes, the contemporary lifestyle is stressful, and yes it is very different from the way we've evolved to live, but it's probably still much more comfortable and enjoyable than any aboriginal circumstance and prehistoric way of life our ancestors found themselves in.