Yeah, uh... you are aware that you can, in fact, run the dragons precisely that way. Have the PCs fight them one at a time. But I'm guessing they feel that fighting a bunch of gold dragons in a row wouldn't be very good game design, and so didn't go that route. That doesn't mean you can't if you want to. But establishing that as the default - that Bahamut always has these allies, and that either the PCs fight them as a group (a terrible, terrible idea) or that they have to lure them away (removing them as a part of the encounter anyway)... yeah, that's just a bad idea, and bad design, no matter how you look at it.