That fits right into the story , but I question the need to eradicate man first , unless it's just one of those "make damn sure it's done" type things.What I read could be interpreted either way I thank you for clarifying more.
It's not so much the need to eradicate man first, so much as it's... the same thing. Talos, Lorkhan, and Humanity are all parts/fragments/aspects of the same thing.
Humans are fragments of Lorkhan, just like the Mer are the fragments of the Aedra. The Thalmor plan is to make it so Lorkhan never existed, which means they need to make it so mankind never existed. The two are the same thing, kind of, so doing one does the other. This will undo creation, since Mundus was Lorkhan's idea in the first place, which means Mer will reform into their pre-creation divine forms.
Talos is a sort of new Lorkhan, basically. If they destroy the world (by destroying or disabling the Throat of the World) while he's still a god, it will just be the end of the world, because Lorkhan, as Talos, still exists and thus has always existed. If Talos is dead and all of humanity is dead, no part of Lorkhan will remain, and once he no longer exists he will never have existed and reality won't just end, it will never have come to being in the first place.
It's all very metaphysical and whatnot, and there's a bit more to it than all that, but that's pretty much it in a nutshell.