I hope the rumor is wrong on the 200mm. I've had mine for less than six months. Part of my buying decision was that it has been on the market less than five years and I expected it to remain current for several more.
I'm some what concerned about losing value, but knowing Canon the new version would cost much more, so I maybe not.
I'm most concerned about obtaining parts and service for my existing lens, once it's been discontinued. I personally would not buy a discontinued lens without a hefty discount, just for this reason.
ALmost NEVER will the previous version decrease in used prices when a new is released. If you don't own one of these, buy it now as the used prices will go up when the new one hits the market (I'm somewhat speaking to myself on that one also)