all those things that can be "bought with bitcoin" are actually bought with dollars that come from the sale of bitcoins to BitPay/Coinbase/etc...
Does it not work both ways? Most of my bitcoins were purchased with dollars, received in exchange for my labor. A more appropriate saying would be, "all those things can purchased with labor or traded for other things using a medium of exchange of dollars or bitcoin".
The point is that most of those "purchases with bitcoin" mean that the customer's coins are sold in the market by Bitpay and the dollars are given to the merchant. Where do the coins come from? More likely from people who already had a stash of bitcoins, than from people who didn't have bitcoins and bought them from the market for that purchase.
Therefore, those businesses are likely to increase supply in the market (by moving coins out of hoards to the market), rather than demand. Basically, they induce holders to sell, and make it convenient to do so. Are they also inducing non-bitcoiners to buy bitcoin? It is not obvious: if you had dollars and no bitcoins, why would you choose to pay with bitcoin?
I did not expect the "using bitcoin kills bitcoin" argument from you, I thought you were a little better than that.