This line of argument has been repeated over and over but it doesn't hold.

When a new company adopts Bitpay and customers pay in Bitcoins (that are converted to fiat instantly) then these coins need to have been bought at an earlier time. You cannot spend Bitcoin that you have not bought before. There is no credit with Bitcoin.

Then, why would it affect the price negatively, when people first buy a certain amount of Bitcoin and then sell it back into circulation, using a Bitpay transaction instead of a market order? It doesn't make sense. Actually the price will be increased, as Bitcoins are withdrawn from circulation for the amount of time they are held, i.e. the period between the buy and the payment.

There is one exception to the above model, however. You may consider it a self-evident part of your argumentation (I'm trying to be polite here), but it is not, so let's discuss it explicitly:

There are long-term holders of Bitcoin that may feel encouraged to sell Bitcoins by Bitpay transaction when new goods or services can be paid this way. As these coins are not taken out of circulation right before they are sold back into the market, they will have a negative effect on price.

So, what's the ratio between these two types of buyers? What are the effects on price? We do not know because there is no data available to make any worthwhile conclusions. If Bitcointalk hadn't gone to 100%-troll-mode yet, I'd ask you to refrain from presenting your wild assumptions as facts. But it's a lost cause anyways. Why am I even reading this?