First, there is no guarantee that the recent upswing in bitcoin prices will last.
The cheaper versions of these cards are already becoming a rarity. Just look at amazon, newegg or tigerdirect. As the popularity of bitcoin mining rises this trend will continue. Quite simply put, I can wait.
There's no guarantee the recent upswing in resell value of 5870s and 5770s will last.
Gamers will be upgrading more and more away from these cards, and they represent the vast majority of the market.