Midgardsormr doesn't help us cause he is testing us and isn't incline to interfere. Its on us to do our bit as Hydaelyn's champion, not him.

Taimat doesn't want to leave and you can't save someone who doesn't want saving. She may also not want to leave what is left of her brood, all the insane dragons that live around her who's minds have been broken by Allag experiments. Its also possible that the Dragons can commune with each other over distance using the Dragonsong.

Regardless, Dragons have a fundamentally different mindset to humans, shaped somewhat by having a vastly longer timespan. The Firstbrood, while not technically irreplaceable in the sense of any Dragon that lives as long as them has the potential to be that powerful, are a historic and social institution within dragon society. When they are dead it will be the end of an age for Dragons.