The tickets IATA are buying will be distributed via BSPs to Travel Agencies. Airlines can (and will) still purchase (and issue) paper tickets. The numbers will be very small in most cases but they will still exist because there are some cases where it is impossible to issue an ET.
As an example; the rules for an ET say that you cannot issue more than 16 flight coupons. Round the World tickets can extend to more than 16 coupons and where this happens a paper ticket will have to be issued.