The Japanese are used to crowded transportation systems. They are building runways and airports in Japan now - A second runway in Osaka and a man-made island airport in Nagoya. The 2nd and 3rd largest cities I beleive. They do seem to have a hard time getting the locals to accept a runway in their backyard, though.
Boeing put their balls on the line with the 747 - I think it's the same with Airbus. If it fails (it won't), they should be able to survive, possibly because of gov. help.
But when I think about it -I only remember seeing double-decker buses in britain?