There will be triple deck plane, planes are getting bigger as population is growing. Airbus and Boeing are already pushing their twinjet to limit, they can’t keep making them longer every time there is demand for more passengers per plane. Like with skyscrapers dealing with congestion in city, they have to make them taller to keep up with demands.
Boeing 747 and Airbus 380 are suffering because of fuel cost since they are quadjet planes which use more fuel per pax than twinjet planes. Boeing 747 was very useful for nearly 40 years when fuel wasn’t so expensive. Airbus and Boeing will have to design a multiple decks plane with two or three engines, it will be likely a blend wing design. Airbus might have overestimated the demand for A380, but there will be demands for it few years later.
About cargo, Airbus 380 isn’t effective at doing that job since it has cockpit on first deck which prevent cargos being loaded quickly or large cargo being loaded like Boeing 747 can. Airbus 380 would require cargos to be lifted into 2nd deck by onboard elevator or crane which would weigh lot after being lifted into first deck from cargo lifter.
Just my 2 cents.