Lou,
Not to pick the nit because I agree fully with your basic premise slow golfers find ways to play slowly regardless of mode of transport, but I would like you to consider lessening the absoluteness of these two statements:
"A slow golfer riding will ALWAYS play faster than a slow golfer walking with one possible exception, when carts are restricted to the paths. A person who cares about the speed of play and his companions' enjoyment of the round will ALWAYS play faster riding than walking."
ANY golfer, fast or slow, will be faster IN A STRAIGHT LINE FROM ONE SHOT TO ANOTHER riding than walking. Inarguable, just like a Ferrari being faster off the line than a Civic. On that we agree.
BUT there are a number of instances in which walking vs. riding pace evens out regularly, even when it is not "cart path only". Cart paths are often some distance from tees, hazards, and greens AND on the opposite side of same from where the ball lies or the next shot will be played. Walkers have the option to go in a more or less straight line, while there may be a long walk to the ball, back to the cart, and to the next shot.
For instance, often walkers will be waiting on the next tee for fast players in carts to arrive because, after putting out riders had to walk 30 yds. back to their cart, then circle the green, park and walk 20 yds. to the next tee while the walker went straight off the back of the green to the tee. Happens all the time.
And, of course, the reality is that when play on a golf course is slow, the speed at which one is able to move from where they just played to where they are going to play their next shot becomes less relevant. The Ferrari, by getting off the line more quickly will simply manage to arrive at the next red light more quickly than the Civic.