That's not an example of someone going from spiritual life to spiritual death.
Branches are a reference to those who have attached themselves to Jesus and are tapped into His life. I cannot see how you can say anything else and be consistent with the text. Jesus was making the point that we must remain in that union to continue to enjoy His life (which is eternal). His life is eternal. We have it too as long as we remain connected to Him. We do not have to perform works to stay in that place. Rather, the relationship of the branches to the Vine is one of continual faith/love dependency.
Jesus said: in John 15:4 Remain in Me, and I will remain in you..."
This verse by itself suggests that His "remaining in us" is conditioned on our choice to also "abide in Him."
Remaining in this active relationship is the only way we can bear fruit. However, in John 15:6 Jesus gives us a warning,
6If anyone does not remain in Me, he is like a branch that is thrown away and withers. Such branches are gathered up, thrown into the fire, and burned.
Here he says that it is possible for a "branch," one who has been joined to Himself, to willfully discontinue that relationship.
The result is this: 6If anyone does not remain in Me, he is like a branch that is thrown away and withers. Such branches are gathered up, thrown into the fire, and burned. "Withering is a sign of the cessation of life" Thus the teaching speaks of how someone, who has been raised to life can fall back into death.
This is the simplest, most straight-forward way of handling the text clearly is an example of someone going from a state of spiritual life to spiritual death.