Besides the non-flammable materials requirement, there are at least two others that make upgrading many old ships prohibitive. After 2010, passenger ships must have at least two completely independent propulsion systems and two-compartment survivability, so if any two compartments are flooded the ship will stay afloat.
Doulos is full of flammable wood, single engine, and has only one compartment survivability. If the only issue was flammable materials, Operation Mobilization, the owner, would probably keep her, especially since they are still busy with the replacement for the Logos II, their other ship.