If you can't reverse your payment (due to say not getting something delivered) then why would any online purchaser prefer to use Bitcoin (and talking about escrows is just silly as people don't use escrows when normally purchasing things with credit cards as they don't need to - you can generally just complain to your credit card provider and the merchant will lose out which is what most consumers prefer).
Bitcoin is fully capable of reversible transactions, with far more flexibility than existing methods such as credit cards, at vastly reduced cost, within a competitive open market. See any of the many many talks on this subject.