Many containerised lower-deck (cargo compartment) crew rest areas are unable to be used for takeoff and landing due to restricted access/escape routes and the inability to evacuate in required time due to need to first climb to main cabin deck.
Therefore the idea for use of "container" modules for permanent passenger seating capacity would require significant new design to enable a number of containers to be joined together and so allow multiple escape routes, and also almost inevitably requiring installation of lower deck emergency external exits in the fuselage and connections to the containers to allow access from the container to the outside of the aircraft.
The major redesign of any existing aircraft to satisfy this certification issue would almost certainly make it unviable financially, if not also structurally.