Can wider acceptance cause the value to fall?
The point you bring up is fundamental to understanding how the bitcoin market operates.

The majority of companies that have grown quickly in the space help merchants accept bitcoin. As you mentioned, these merchants generally end up selling bitcoin for fiat currency. Companies that provide bitcoin payment processing have grown more quickly over the past year than consumer wallet companies (which help consumers purchase bitcoin). Consequently, the bitcoin price has experienced downward price pressure over the past year, since we find more market participants selling bitcoin than buying it.

As regulation and banking becomes more attainable for bitcoin businesses over the coming year, we should see consumer wallet companies grow, which would lead to more people buying bitcoin. Until then, expect to see continued downward pressure on the bitcoin price.

PS: I'm the CEO of bitcoin exchange Coinsetter.