It is important to note that most of dealers who accept BTC as a means of payment are using the intermediary services, and they convert that coins to fiat. That actually means that all that spended coins only increase the supply on the market, which means price will not go up because people pay in crypto.
The impact is negligible with how payment gateways use different exchanges to pull out fiat, which directly results in a situation where their sales are distributed amongst several orderbooks and for that reason don't directly impact the market.
If you also take into consideration that most of the payment gateways actually prefer to hold their coins, then you'll realize that there is little to no selling pressure going on. What they do is short a small percentge of their sales to preserve their fiat value in case the price goes down. On top of that, BitPay needs coins as well, so they won't ever sell everything they have because of how certain merchants prefer partly BTC settlements instead of fiat in full.
Another thing is that if they do end up selling to take out fiat, they won't blindly dump their coins at once but will patiently wait for orders to pop up capable of absorbing their sales.