Ideally we would like to buy things with bitcoin, and that the seller would hold bitcoin instead of selling it. Of course this can only happen if mass adoptions occurs. Let's imagine a bakery, and that we buy bread with bitcoin. It would only make sense for the seller to hold his bitcoins, if he could also buy the raw material to make the bread with bitcoin. So only when the entire industry is allowing payments with BTC, then we could see sellers keeping their coins.

This will take time to happen, but even if it doesn't, and bitcoin is just a payment method like PayPal, where people recharge their wallets to make payments, I don't think it would be bad for the price, but quite the opposite. It would not be the ideal situation I've described, but a lot of people would still be buying bitcoin to reload their wallets and continue to make payments with it. So we would have a lot of people buying and selling bitcoin at the same time and this would result in an equilibrium, so the price wouldn't really move. It could be moving up, because more people were using this "new" payment method, but I doubt it would go down.