With the average person wearing mostly insulating clothing, where is the path to ground, and where is this non-isolated supply?
Shoes act like excellent capacitors, so pass AC quite well.
Plates something like 30cm x 10cm, separated by a centimetre or more of dielectric don't produce a very high capacitance. Your risk is much more of it being a hot sweaty day or a rainy day, and moisture linking you to the floor. Even if you do couple to the floor, how many floors are significantly conductive? If you stand in the average shoes and grab a live wire you feel nothing more than a slight tingle, due to capacitive coupling. If you touch a live wire and get a serious shock its probably because your other hand is touching something conductive and grounded.
Not from my experience. Even upstairs on wooden floor, I've received a shock strong enough for a severe muscle contraction - definitely not touching anything else. Trainers and shoes are often reasonably conductive to help stop people building up static.
Maybe you should do a video of you grabbing a live wire
I build up plenty of static in trainers, but even if some are treated they will be static dissipative, not conductive. Their impedance will be huge. In any case, standing on a wooden floor would isolate even sweat soaked shoes.
The human body's capacitance to the environment is tens to hundreds of picofarads. It depends on the body and what its doing. Lets take the figure used in most things needing a human body model, which is 100pF in series with1.5k ohms. An adult standing up, wearing typical shoes, and touching nothing should be in this range. 100pF at 50Hz has an impedance of about 30M ohms. At 220V this will pull about 7uA, and cause a tiny tingle. How tiny? Well the Y capacitors in a power supply are generally bigger than 100pF. If you touch the isolated and safe output of the power supply, and something grounded, you will pull more current than standing up in typical shoes and touching the live wire of 220V mains.
If you felt a serious jolt you had another path to ground. Its really easy to miss them in the heat of the moment.