If the Thalmor attack all the way, they will be killing humans and Talos worshippers at the same time. There are two things that might happen if they do this.
Firstly, it might be possible for the remaining humans to increase their worship of Talos in hope of salvation. This might work out and make the Concordat look the worse choice. Talos might even incarnate and help win the war and rebuild a new Empire.
Secondly, it might be possible for the remaining humans to lose faith in Talos. The possibility of such a thing is evident in how people lost faith in the Nine Divines when the Daedra attacked during the Oblivion Crisis, even Martin Septim himself. If this happens, Talos and the other gods will weaken as the Thalmor invade. He will not be able to save the humans and eventually Talos and Humanity will both be destroyed in a single sweep by the Thalmor. This makes the Concordat look like the better option.
Firstly, it might be possible for the remaining humans to increase their worship of Talos in hope of salvation. This might work out and make the Concordat look the worse choice. Talos might even incarnate and help win the war and rebuild a new Empire.
Secondly, it might be possible for the remaining humans to lose faith in Talos. The possibility of such a thing is evident in how people lost faith in the Nine Divines when the Daedra attacked during the Oblivion Crisis, even Martin Septim himself. If this happens, Talos and the other gods will weaken as the Thalmor invade. He will not be able to save the humans and eventually Talos and Humanity will both be destroyed in a single sweep by the Thalmor. This makes the Concordat look like the better option.
The fact is, to push the attack is costly and risky. This way, there's no risk, very little cost. All they have to do is be patient. It's a win-win for them.

