it may not even be related to feet/shoes, coming in contact with the rubber....
If you look under the door itself, it looks like it is rubbing on the molding when the door closes, and opens....the opening of the door could be rubbing, or 'pulling' more, causing wear and friction on the rubber molding, and in time, this will happen.
Incidentally, mine is a 2013, and I never, ever put my feet on any part of the sill, or the rubber molding....and mine shows minor, minor, wear.
May also be related to the weather climate of the cars....colder climate may make the rubber harder, and the door opening and closing may be harder on it than in hotter, drier climate.
maybe the molding needs some kind of slippery moisturizer put on the molding, and under the door, where the 2 comes into contact with one another?