Wouldn't the fraction of people paying with Bitcoin be a subset of the people that didn't commit fraud with credit cards? You're just moving a very small number of honest buyers from credit cards to Bitcoin.
You not going to have a person not defraud you because you provided the option of Bitcoins. If you have transaction that you deem risky, require a bank transfer, that will make it easier for the criminals to go commit fraud somewhere else.
Cash has no chargebacks, which is why merchants will often offer a small discount for using cash (or charge a .50 fee) when using a credit card. Bitcoin is just like cash, but digital. Some merchants even offer discounts for customers who use bitcoins since the risk is mitigated, as with physical cash.
You not going to have a person not defraud you because you provided the option of Bitcoins. If you have transaction that you deem risky, require a bank transfer, that will make it easier for the criminals to go commit fraud somewhere else.