The two Presocratic schools of thought that are present in this section are the Pythagorean school, of which Cebes, Simmias, Phaedo, and Echecrates are all members, and the Heraclitean school, named for its founder Heraclitus. The Pythagoreans were noted physicists, and held that nothing in the universe comes to be or ceases to be spontaneously. Apparent generation and corruption are just surface changes in an unchanging, underlying substance. Thus, the Pythagoreans would be eager to agree with Plato that nothing is ultimately created or destroyed at birth or death, but that rather there is just a change where opposites come into being out of one another. Heraclitus also maintained that things come to be out of their opposites, and that the balance between opposites is crucial for the existence of order in the universe. He also maintains that everything is in constant flux and that the only constant in the universe is change. Plato is resisting this extreme, arguing not only for the constancy of the soul, but also of his unchanging Forms.
The problems in this section arise from the shifting question of what Plato means when he discusses opposites. As he first introduces them, it seems he is talking about comparative opposites, such as bigger and smaller. Something that becomes big is bigger now than it was before: before it was smaller. It is presently big only in comparison to what it was before. But then he shifts from comparative opposites to absolute opposites. There is no discussion of "more dead" or "more alive," there is just "dead" or "alive" to choose between. Someone that is alive now is not said to be alive because they are less dead than they were when they weren't alive. There are no gradations of being dead or alive as there are with being big or small, and it is less clear that in the case of absolute opposites, each one must come into being out of its opposite.
There is also the question of what is dead and alive. For Socrates' argument to work, he must conclude that the soul is what goes through these states of change, thereby proving that it does cohere even after death. But is it the soul that dies? Surely, Socrates wants to argue that the body dies while the soul lives on, free from the body. But if it is the body that dies, the argument breaks down. It would be absurd to suggest that living bodies come into being from dead bodies, and besides, this would not help us to establish the immortality of the soul. Perhaps Socrates might have chosen the opposing pair "embodied"/"disembodied" to describe the progress of the soul between its opposites, rather than "alive"/"dead."
Finally, we might consider Nietzsche's words: "Life is not the opposite of death; it is an exception to death, and a very rare one at that."