If they are saving card payment processing fees, that should balance off the conversion fee. Btc is already one of the most liquid assets in the developed world (judging by volume and bid-ask spreads), even more liquid than most traded commodities and currencies (outside major pairs).

So they should be able to pass some of that savings from credit card companies to the consumer. And as more merchants adopt, buy, sell, convert, the liquidity pool gets bigger further reducing the bid ask spread. That is how adoption spreads.

Eventually, BTC will be so liquid that it becomes the denominator, not numerator.