Great question! I don't think we would see replacement of dead neurons to any significant degree. However the process of neuron death is gradual. Neurons are thought to die back from the junction with the muscle. The neuron withdraws from the muscle, the nerve fiber shortens, and then finally the body of the neuron, located in the spinal cord, dies.
Now as soon as the connection is lost with the muscle, the neuron is as good as dead, as far as we are concerned. It is no longer doing its job of carrying signals to the muscle. We have all the same weakness and loss of function from these dying neurons as from ones which are completely dead.
The point is that any therapy which can halt nerve death and stop this process of destruction might well also allow damaged nerve cells to heal. Totally dead cells are not coming back; they basically get eaten by macrophages, the vacuum cleaners of the body. But cells which are dying, which have lost the connection to the muscle but with the cell body intact, might well recover. Once they get healthy then they could start to regrow the nerve fibers and reattach to the muscles. And this would mean restoring some partial functionality.
So in general I would hope that a treatment to stop the disease process would also let patients regain some of the strength and movement they had lost. Then possibly other therapies with stem cells might eventually replace the lost neurons and give further improvements.
