I don't think paying with bitcoins has any effect for bitcoin holdings. Because the money I pay with bitcoins I can buy those bitcoins with fiat money if I want. But there may be some variation as volatility purpose. Many people emotionally say they don't want to pay with Bitcoin. But he can never invest bitcoins beyond his limitation. They must be spent every day to manage their life. Which you can't hold even if you want. So I see no problem in paying in bitcoins for what is outside my holdings.
But if the market is in a down trend then paying in Bitcoin can be a matter of loss. At least if a payment provider feels it has nothing to lose he can pay in Bitcoin.