A few problems:

-The dragons would probably just fly away from something like Black Rose.

-Dragons aren't attached to particular mortals. They are largely apathetic to the conflicts of man, and as such rarely intervene (Midgardsormr's unexpected rescue at Silvertear being the first, last, and only instance thus far).

-The Dravanian Horde (or at least Nidhogg's brood) could have wiped out Ishgard any time he wanted. Nidhogg's goal wasn't to destroy Ishgard, it was to make them suffer.

-We can't count on Midgardsormr for aid, as he used the last of his barely-recovered energy to break out and save the Warrior of Light when Omega trapped and tried to kill them in an attempt to awaken their latent power.