Are you saying Gold Dragons are dependent on another species for their evolution and survival.
I just do not see it.
Well you probably haven't seen a gold dragon around lately either.
There's any given number of justifications for the existence (or lack of) for any fantasy species in a fantasy world. Whether or not Bahamut ordered all non-good gold dragons to be hunted down and slain, and whether he was successful, is up to the DM.