Yeah! You've got a point there Dogwood. Thinking about medieval shoes, my assumption is that they were not much more than mocassins, with thin leather soles. My deduction is this, shoe wearing was as much for keeping your feet clean and, in winter, warm. these thin soled shoes were probably perfectly healthy for your feet.
Good deductions!
Based on all my reading -- and now personal experience a couple of weeks in on my barefoot experiments -- your deductions are exactly on the mark. The thing that weakens the feet is the padding, the arch supports, insoles, tight bindings of the toes and ankles, etc. of modern shoes. If you can't go barefoot, wear shoes with as little structure as possible.
The real culprits are probably fashion and hard paving. I think that by the eighteenth century being barefoot was a sign of poverty.
From what I can determine from studying this, this isn't exactly the case. In Europe and the UK and the US, people in rural areas were often barefoot well into the late 19th century, although they would wear shoes for church, in the winter, etc.
In the cities, largely because of the unclean roadways, people routinely wore shoes unless they were destitute beginning in the late 17th century.
In the colonial Americas, almost everyone went barefoot most of the time (especially in the South), including the likes of Thomas Jefferson when he was at home. It was just part of the scene.
Here's an 18th century quote that might tickle you:
"It would be a great novelty for a Londoner to see one of these congregations -- the men with only a thin shirt and a pair of breeches or trousers on -- barelegged and barefooted -- their women bareheaded, barelegged and barefoot with only a thin shift..." Charles Woodmason, touring the Carolina backcountry, 1766.
I guess we Yanks were always on the casual side
Of course, large numbers of colonists on the frontier often dressed like Indians too, but that's another story...