If you look at pictures of people in 1940s and 50s Malta you will see plenty of people - most of them belonging to the working class obviously - going about their business barefoot. Shoes were expensive for people like that - remember that not everyone was as well off as - say - the Americans (though even that is generalising - just look at the Great Depression). Europe was ravaged by two world wars which left many many people destitute. My grandfather used to say that sometimes they would own a pair of shoes but only wear one shoe at a time ... to make them last longer I guess. I'm sure this is still the situation in other, less lucky, parts of the world.
I'm sure your grandfather was joking, but I'm equally sure that conditions were as you say they were - people without shoes
Why and how would they not be wearing shoes if they had the brain capacity to know what a shoe was and how to utilize it and when and why?
The answer is because they either didn't have the time, skills and materials to make shoes for themselves or they didn't have the goods to barter for shoes or the money to pay for them. Consider, an African in Somalia may well have the brain capacity to know what a computer is and how to utilize it and when and why, but the reason they don't have one is covered in the previous sentence.
Pretty well every society tends to "pass back" its standards to earlier societies, this is why mediaeval and Renaissance painters paint figures in Biblical scenes dressed as if they (the figures) were living in the middle ages, because that was the norm for the painters. There is no definite proof either way about shoes, but there is plenty of evidence to suggest that it was more likely that the majority of the population in mediaeval times didn't wear shoes as we would recognise them; if the majority of the population wore any sort of foot covering, then it was most likely something they had made themselves, e.g. rag "shoes", bark "shoes" or some sort of sandal.
And what, pray tell, is this assumption based upon?
To be frank, this is a condescending and arrogant statement.
On the question of classes in the middle ages, there were a range of these, the king obviously being at the top and his feudal lords below him to greater or lesser degrees. Below this were the rest of the population and in a system based on the ownership of land, these people owned no land, although they could have rights to land, e.g. in the form of the strip system for agrarian farming and the right to graze animals on the village common or to let pigs roam the woods for mast. They could be closely under the control of their lord, even to needing permission to leave the village. Over time, the system gradually changed with the appearance of a small middle class who provided services or made goods to trade rather than relying on the land for a living. (Although these would often still retain links with the land their family worked in their village, even though they lived now in a town.) But the lord still had influence to over them. It was the Black Death in 1349 which provided the circumstances for radical changes as the fall in the labour supply and the disruption in the ownership and management of estates meant that the links between the lord and his "vassals" was weakened severely or even broken. The landless peasant now could have the opportunity to get his own land or hire himself out as a paid labourer; the better off peasant had the opportunity to acquire more land, and so on leading to the appearance of a wage earning class of former peasants working on the land or in the towns and a middle class of wealthier farmers and people producing and selling goods and personal services.
Obviously, this is a summary of a complex development which was spread over a period of time.
