First and foremost, Bitcoin transactions are irreversible. Unlike purchases with Paypal and credit card, Bitcoin transactions are forever one-way tickets, so merchants are completely able to avoid the hassle by the name 'chargeback'. If you don't pay a fee, the only thing we know is it may take a while for your transaction to get confirmed. We don't know how long your transaction can get confirmed if no fee is paid. The only problem that merchants and retailers may face taking payments in Bitcoin is what Bitcoin creators defined as 'double-spend'. 'Double-spend' in some circumstances may happen but it can easily be avoided too.
Transaction fee is and will never be an issue for Bitcoin in the sense that merchants may be willing to take that part for their buyers. After all, Paypal and credit card companies all require expensive fees too. Why do people still use Paypal and plastic cards every day? and who pays the fees when you use them?