The death of the body and the disappearance of existence itself are not the same thing.
Krishna says, "There was a time when I did not exist, and there was a time when you did not exist, and there was a time when the kings who are here now did not exist." And he continues, saying that in the future as well, none of us will cease to exist.
Arjuna is afraid of the complete annihilation of his relatives who are on the battlefield. In contrast, Krishna distinguishes between the death of the body and the disappearance of the soul. He explains that both Arjuna and himself, as well as all the kings, have existed in the past and will continue to exist in the future. He begins to teach from the perspective of the permanence of the soul, stating that there is no need to lament solely based on the fear of the complete loss of existence.
Japanese Translation from the Geeta Press Edition - Chapter 2, Section 12: "There was never a time when I did not exist, nor you, nor they."
In the previous section, Bhagavan Shri Krishna told Arjuna that he was lamenting for Bhishma and other relatives, who are not worthy of being lamented. Naturally, a question arises as to why they should not be lamented. Therefore, the Lord first establishes the eternity of the soul, and from the perspective of the soul, it proves that lamenting for them is not wise.
Sanskrit original text
न त्वेवाहं जातु नासं न त्वं नेमे जनाधिपाः ।
न चैव न भविष्यामः सर्वे वयमतः परम् ॥ १२ ॥Romanization
na tv evāhaṃ jātu nāsaṃ na tvaṃ neme janādhipāḥ |
na caiva na bhaviṣyāmaḥ sarve vayam ataḥ param || 12 ||Actually, there has never been a time when I did not exist. There has also never been a time when you did not exist, nor have these kings ever ceased to exist. Furthermore, there will never be a time when all of us cease to exist. (12)
In this section, the speaker establishes the eternal nature of the soul. And it shows that people, including Arjuna, who are afraid of losing something, as well as Arjuna himself, and even Bhagavan Shree Krishna, have never existed in the past, present, or future, at any time.
They existed before their current bodies were born and appeared. And they continue to exist even after those bodies disappear. The destruction of the body does not bring about the destruction of the soul.
Therefore, Arjuna should not lament for his relatives because of the fear that they might perish.