In the dawn, there were a great many spirits, and these spirits were immortal so long as they properly navigated change, the fluctuating conflict between IS and IS NOT. Then Lorkhan (aka Shor or Shezzar) came around and proposed a plan to make this navigation easier. The other spirits, tired of the way things had been going, agreed to his plan. But oh no! It was a trick! The spirits who participated in Lorkhan's plan were trapped, and died. The greatest of these spirits had been anchored in place, and remained dead, their corpses being used to support the prison of the lesser spirits. These lesser spirits became Mortals.
Some of these mortals stayed in place, built cities, and tried to remember what it had been like to be immortal and free. Others wandered the world, and came to accept their fate. Those who stayed in place became the elves, long-lived, eternally healthy, and capable of great mythic constructions. Those who wandered became short-lived barabaric men, saved only by the grace of their prison warden, who loved them dearly.
The Altmer wish to destroy the prison, and make everyone immortal again. Mankind wants to have fun in the cage, and maybe see if they can't learn some lessons from their incarceration. To do this, they need to completely destroy Lorkhan. The strongest aspect of Lorkhan is Talos, and so their first step is destroying him. Once they succeed in destroying Lorkhan and freeing all spirits from the prison of the mundus, mankind, defined as "the short-lived races who support Lorkhan and enjoy life on mundus" will be impossible. You can't be a short-lived immortal, you can't support something that doesn't exist, and you can't enjoy life in a place that isn't. This will, from the Altmeri perspective, actually benefit the souls of humanity. Everyone will become immortal when the mundus is destroyed.
I have to disagree here, the Altmer have the numbers and experience? From what I understand, the Altmer have a slower birth rate than man kind and for the most part, the Altmer haven't half the combat experience of the manish races. History hasn't been kind to the Mer via military exploits.
The Altmer actually have a vastly superior breeding capacity. Their breeding rates are kept in check by their eugenics program, but were it not for that, their normal fertility, combined with their massive period of fertility (several hundred years vs, several decades) would allow them to flood the world with golden children. Of course, the Thalmor will never do this, but it is theoretically possible. And it was only during the 4,000 year period of Mannish Empire that the men have had a good record against Mer. Before and after that, Mer kicked massive butt.