That's the problem: there's no way for that to happen. Imagine Amazon started offering 5% off with Bitcoin. Visa tells them to stop it or they'll lose their account. What do you think comes next?
Today (at least in the US) merchant agreements can't prohibit discounts for alternatives they can only prohibit increased cost for credit cards.
Here is an example of gas stations offering cash discounts:
http://www.creditcards.com/credit-card-news/gas-discounts-for-cash-1275.phpNow the one con in the article was that cash is inconvenient. Hmm imagine if someone made a digital cash like system so consumers wouldn't need to carry around cash to get discounts? Someone should think of that.
I am not saying this will happen tomorrow or that it will EVER happen but VISA and the other CC demand ever increasing profits. Those costs combined with chargeback fees, and losses due to fraud do add up. Credit cards aren't free for the merchant and thus aren't free for the consumer either.
It was asked why a consumer would use Bitcoin over CC. There is your answer. Cost savings. Now trust matters more in Bitcoin but I would have no problem trusting many of the places I shop online with Bitcoins. I can't remember the last time I made a chargeback but I pay a hidden cost of 5% to 10% tacked onto every item even when no chargeback occurs. Some transactions are risk and you can always use CC there (or some reversable system built on top of Bitcoin).