Burt doesn't have to live. He can die. To me his story is done. He and Saul are even, he could have stayed in the hospital, but he wants to fight to protect the colony. With the walls being rushed, and him being weaker than even Lizzy is, he is going to fall behind. I'm not saying hes going to get taken out by the front line, but he is going down at some point. Hopefully taking TOWTM with him.