With this shloka, Krishna seals the entire teaching on the soul's nature. The soul dwelling in every body — not just human bodies, but every living creature — is eternally beyond destruction. Therefore, Arjuna, you should not grieve for any being.
Notice the sweep of that phrase: 'sarvani bhutani' — all beings. Not just Bhishma, not just Drona, not just the warriors on the field. Every creature that breathes, moves, or grows carries this same indestructible essence. The scope is breathtaking. What Krishna began in 2.11 with a pointed rebuke — 'you grieve for those who should not be grieved for' — he concludes here with a universal principle.
This is the closing statement before the argument shifts. Everything from 2.11 to 2.30 has been about the nature of the soul. From 2.31 onward, Krishna will speak in a completely different register — the language of duty, honor, and a warrior's dharma.