Good remarks.
I also am a member on a SA site
www.outdoorphoto.co.za where members shoot with Canon/Nikon exclusively and to be quite honest the main reasons they use those brands is due to the availability of lenses, ff bodies new and 2nd hand and rentals.
So for them it is way easier to trade older for newer which basically is impossible with Pentax,
Thus they really don't lose much on the investments as there are a multitude of willing buyers/sellers in the market place - something I hadn't really considered in the past.
Right now one can pick up a 50D for half the price of brand new as many are wanting to fund the new 7D.
If I wanted the K7 I would have to buy full retail as I would not be able to trade the K10D nor the 20D (if I owned one)
Dylan