Know this, Krishna says: that which pervades this entire universe is indestructible. No one has the power to bring about the destruction of the imperishable. The soul is not a small thing trapped inside a body. It pervades — it extends through everything, everywhere, at all times.
Think of the space inside a clay pot. Break the pot, and what happens to the space? Nothing. The space was never contained by the pot in the first place. The pot gave the appearance of boundaries, but the space within was always connected to the space outside. The soul is like that space — no vessel can truly hold it, and no breaking can destroy it.
The word 'avyaya' — imperishable, without decay — rules out even gradual deterioration. The soul does not age, does not weaken, does not diminish over time. It is not subject to entropy. What Arjuna fears on this battlefield — the loss of his elders — cannot touch what they truly are.