It seems they could fit 3 and 3 seating in that lower section. Plenty of room down there. Windows would be problematic. But if they put in a low-cost "Galley Slave" class, I'm sure a lot of people would take them up on it.
Apart from the practical problems with egress, etc., the long haul airlines make much more money putting freight in that slave galley than they could with paying passengers.
Why would airlines want to do that? They make more per pound of freight than they do from carrying passengers. One reason why the A380 isn't a very good design is that with two full decks of passengers, there isn't much room left in the cargo holds for revenue cargo after all the passengers' baggage is loaded. One of the advantages of the proposed 787-10 will be that two of them have more passenger capacity than one A380 with lower cost per seat mile and 180% of the revenue cargo space.
The 747-8 will be able to carry several more standard LD-3 cargo containers compared to the 747-400. It also is able to get longer range without using any fuel tanks in the cargo hold due to increased fuel capacity in the updated wings and improved performance of the engines.