First, I'd like to speak on something unresolved. While we mostly agreed that the helium air bladder would not be enough to get a dragon airborne, would it be possible for a quadrapedal animal with an overproportional wingspan, near hollow bones, and an airbladder filled with hydrogen (Twice the lifting power of helium) to fly?
Meanwhile, I must come to Hapsburg's defense in that, while he did say that he believed in dragons, he never tried to force this on anyone else. Also, I would say that while we may need proof to prove dragons' existence, we also need it to prove their lack of existence. A dragon is commonly viewed as the great winged, fierbreathing behemoth of European mythology, but it does not have to be such. As your dictionary listing said, generally viewed as such.
Asian dragons in particular ar very possible, at least the physical aspects. As for the European one, anything is possible. As was previously mentioned, the platypus was viewed as a total hoax by nearly all of the British science world until a living one was catalogued. Meanwhile, combinations of monkey and fish bodies that were known hoaxes spawned much bogus scientific research and even today are more well regarded than dragons next to bat men and women that have had children with aliens or bigfoot in the tabloids. All history of humanity is filled with views of how ludicrous our viewpoints have been. Maybe in years to come people will see how stupid we were to believe or disbelieve in dragons, but until an absolute answer has been found, everyone is entitled to their belief and no one should be insulted for it.