By the same token, no one can refute "Bitcoin could be at $30 in a few months!" since both statements rely on subjective and selective data.

This is a gamble, pure and simple, and anyone who thinks they have a "foolproof system" is the one who's going to be hurting soon.  Play this like any other market -- take profit when you can, don't invest more than you can afford to lose.

There is a second gamble here, though:  the VLIW design could change for worse or for better, like you said.  Consider this: if the 7000-series changes in such a way that they emulate NVidia's approach (better actual D3D/OGL performance at the cost of lower hashing performance) then the 69xx cards you now have start to go UP in resale value, since it becomes known that there won't be anything in the near term to replace them.  This is, again, a gamble, you are betting that the future will turn out one way or the other.