People have been saying this for a very long time now and it hasn't happened. I'm not saying there haven't been instances of some of it, but it's not at all inevitable, like you are making it sound. There are people out there running AMD 5000/6000/7000 cards that have made enough money to make being a computer enthusiast, not just free, but profitable. At $9/wk you would pay for a 7970 in a year. Basically, free state of the art card.
I've been reading these posts since the 5000 series and it's proven to be nothing more than fear mongering. Frankly, I wonder the motivation for it? Could it end tomorrow? Sure it could. Everyone knows that though. It doesn't mean that it will end tomorrow, and if it does anyone who bought an AMD card, because of bitcoins, will somehow be worse off than someone who owns an nVidia card.