I'm not sure exactly how technology would become integrated into the whole 'journey through the afterlife' thing, but I think your observation about them reflecting technology in the Land of the Living is sound.
As you say, the appearance of airships would imply the technology in the Land of the Dead reflects what's currently available in the Land of the Living in the era the game's set.
Perhaps the demons are the key.
We only see those actually dealing with the technology and building it, so perhaps they are spiritually intertwined with the Land of the Living and whatever's invented there is passed through them to build in the Land of the Dead (tee :~). Thus, the Mayan Mechanics could very well replace the Number 9 with something better (they presumably built it since they built the vehicle for it) once the technology exists in the Land of the Living.
The only question then would be: what about people who actually knew how to build these things in the real world, and could very well carry on inventing better things in the Land of the Dead? Is this knowledge stripped from them, forcing them to live with whatever the demons produce?