Who says we are going to fight Alduin at all? Why couldn't we just stop him from coming?
I dont know, no one does but I dont' see em spending so much time on dragons only for us to seal away Alduin with no big final fight. We'll see.
The Dragons want to go home, and the Men & Mer want to live.
No one is technically "right"; it's a game of survival.
Well, men and mer are currently in a civil war of all things. The world has gone to hell in a hand basket. The dragons/spirits or whatever they are probably could have survived just sitting on the sidelines like they have been. Seeing the men and mer squander their gift of life and freedom by fighting each other over something so petty probably is the last straw in the line of things. Some of the spirits may have even helped us when Alduin attacked, but seeing us throwing away the gifts we've been given probably was the last straw for a lot of them.
Technicaly men and mer are in the wrong for not using the gifts they were given, instead throwing them away by fighting and killing their fellow brothers over power. But if the warring factions can unite under the dragonborn as a form of redemption then some of the spirits may change from Alduin's side and come back to our own. With the help of some of the lesser spirits, the dragonborn could in theory fight toe to toe with Alduin. He is the god of time...but faith holds a considerable amount of power in Mundas. With the faith of men and mer, and the help of a lot of spirits, a dragonborn possibly could force the god of time back.
Who knows though. It is a fight for survival for men and mer, and that might be enough to halt the fight over power and combine forces to fight a common enemy. For the dragons, its less of a fight for servival...and more of a fight to go home cause their pissed off that these lowly lifeforms which they helped give life to not only get the freedom they wish for but are not even using it.