Joseph,
“Was exercising and mental health concerns non-issues from the time of Sarah Imainu through Chazal and the Mechaber of S”A that all cited this restriction, and they only became relevant for the first time in history in recent years?”
Stop for a minute and think. Today we have cars that allow us to travel for miles while sitting down. We have washing machines, so we don’t have to schlep clothing down to the creek and use a washboard. We have electric stoves and running water, so no more bringing firewood into the house and trips to and from the well with heavy pails. The need for exercise has always existed, but in our generation the opportunities to get it via our daily routine have all but disappeared.