Those of us with substantial holdings will not deplete them by buying stuff at a low exchange rate. We didn't get where we are by buying high and spending low, so price needs to go up gradually and then we'll share with merchants. Overstock bitcoin sales went down because the exchange rate went down. If it goes up, more bitcoiners will buy stuff there and then more merchants will jump on board.
If Overstock sales went down it's because the initial rush of bitcoin buyers has passed.
And anyone with substantial holdings of BTC almost certainly acquired it more than a few months ago (ie when BTC was < $150). In the context of the gains made, the exchange rate has barely moved.